On the evening of 9 June 2017, as bewildered Conservatives struggled to comprehend the loss of their majority in the general election, Kensington turned red. Labour’s victory in the final parliamentary seat to declare was slight – 20 votes – but historic. Since its creation in 1974, the UK’s wealthiest constituency had been the sole property of the Conservative Party. As word of Labour’s victory spread, cheers reverberated around London; WhatsApp messages between party staffers and journalists carried a single word – “Kensington!” – an expression of celebration and incredulity.

Kensington: the seat’s very name was synonymous with Conservative supremacy: Alan Clark, Michael Portillo, Malcolm Rifkind. On election night, as the first cracks appeared in this edifice, Rachel Johnson, the sister of former London mayor Boris Johnson, trembled like a French aristocrat confronted by insurgent Jacobins. It gave her “shivers”, she told CNN, to think that “the home of Kensington Palace, Peter Pan, Diana, Princess of Wales’s old house” had fallen to the Corbynites.

And yet in London, this was no freak occurrence. Labour took Enfield Southgate and Croydon Central, both of which it had not won since 2001, and Battersea, where its last victory was 2005. Jeremy Corbyn’s party turned marginals into safe seats and safe seats into fortresses (winning 49 of London’s 73 constituencies). Across the capital it won 54.5 per cent of the vote (to the Tories’ 33.1 per cent), a share reflected in the kind of outsized majorities usually seen in one-party states. When exit polls for the recent Russian presidential election suggested Vladimir Putin had won with 73.9 per cent, Labour MP David Lammy tweeted tauntingly: “Must try harder next time, Vlad, I managed 82 per cent in Tottenham.”

On 3 May 2018, the day of the local elections, London’s political geography could be further transformed. Labour is hopeful of winning Barnet council (last held by the party in coalition with the Liberal Democrats in 2002), Wandsworth council (Tory-run since 1978) and Westminster council (which Labour has never held).

For the first time, the capital’s one million EU citizens (who could not vote in the general election or the 2016 EU referendum) will be empowered to punish the Conservatives for Brexit. Should the Tories lose 93 councillors – less than three in each of the capital’s 32 boroughs – they will fall below their previous nadir of 511 in 1994.

London was not always a “Labour city”. For eight years, from 2008-16, City Hall was occupied by a Conservative mayor: Boris Johnson. At the 1987 and 1992 general elections, the Tories finished further ahead of Labour in London (by 15 points and 8.2 points, respectively) than they did nationwide (11.4 points and 7.5 points). Since 1997, the capital has gone red at every election. But even in New Labour’s pomp, fewer than half of Londoners voted for the party (49.5 per cent). With 54.5 per cent, Corbyn comfortably surpassed this peak.

London now rivals the north-east of England (where Labour won 55.4 per cent) as the party’s strongest redoubt, and is unrivalled as the capital of Corbynism: home to the left triumvirate of Corbyn (Islington North), John McDonnell (Hayes and Harlington) and Diane Abbott (Hackney North and Stoke Newington).

In Martin Amis’s 1995 novel The Information, the protagonist Richard Tull observes of his rival Gwyn Barry: “Of course Gwyn was Labour. It was obvious… Obvious because Gwyn was what he was, a writer in England, at the end of the 20th century. There was nothing else for such a person to be. Richard was Labour, equally obviously. It often seemed to him, moving in the circles he moved in and reading what he read, that everyone in England was Labour, except the government.” In London in 2018, it sometimes feels as if everyone you meet is Labour.

When I first conceived of this piece late last year, a title immediately suggested itself: “Red London”. And then on the evening of 26 January, as I walked home through Mile End Park, there it was: the phrase, spray-painted on to a wooden plant box and twinned with a Soviet-style hammer and sickle. In the distance loomed the capitalist citadels of Canary Wharf. How, I found myself asking, did this juxtaposition occur?

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Emma Dent Coad, the first Labour MP for Kensington, is not a Blair-esque “centrist”. She is a Corbynite republican whose political hero is Tony Benn. “I’ve had the same politics forever, I stuck it out loyally through the Blair years, Iraq, Ed Miliband, God help me,” Dent Coad, 63, recalled when we met at Caffé Concerto on Kensington High Street. “Our current leader is far more in tune with my politics, which is creating fairness: you put in what you owe and you take out what you’re due – it’s that simple. People from all classes and income brackets believe that.” Between Corbyn’s 2015 election as leader and his re-election in 2016, membership of the constituency party rose by more than five times to 1,500 (with 546,110 members, Labour is now one of Europe’s largest parties).

Kensington, Dent Coad said, “is a microcosm of London”: poverty co-exists with plenty. In the borough of Kensington and Chelsea, the average house price is £2m and the average salary is £123,000. Yet 29 per cent of children live in poverty and 5,761 people are homeless (Kensington is, by some distance, the most unequal London borough). Golborne ward, which Dent Coad continues to represent as a councillor, is the poorest in London: 80 per cent of households are classed as deprived, half of children are impoverished, and life expectancy has fallen by six years since 2010. The borough serves simultaneously as a safe-deposit box for the super-rich. Kensington High Street is lined with empty luxury flats whose owners reap the rewards of London’s swollen property market (there are 1,200 long-term empty homes in the borough, and 9,300 second homes). In Ashburn Place, Gloucester Road, seven out of ten properties are empty or second homes. “You have people who are so rich that they can park their £500,000 car and if it gets a ticket or gets towed, they just leave it, they can’t be bothered,” Dent Coad said. Meanwhile, “there are people literally sleeping head to toe with their children in the north [of Kensington]”.

On the night of 14 June 2017, such Dickensian disparities were displayed in apocalyptic fashion. The fire at social housing block Grenfell Tower in north Kensington – the worst blaze Britain had endured for a century – claimed 71 lives.

For those unable to comprehend Kensington’s embrace of Labour, the days that followed were painfully instructive. Theresa May’s refusal to meet and comfort survivors publicly made her appear a stranger in her own land: Queen Elizabeth II after the death of Princess Diana; President George W Bush after Hurricane Katrina. The inert Conservative council, meanwhile, was more concerned with evading responsibility than providing relief. “I can still hardly think about it to be honest,” Dent Coad, who was at the scene of the fire, told me. “We’ve all had counselling, me as well, it still makes me cry to think about it.

“Sajid Javid [the Communities Secretary] stands up in parliament and says they [the survivors] have had their six-week counselling and now they’re on the mend – they are not on the mend… There are people right on the edge, it’s horrible. They tell me everything because they trust me and I hear all their darkest thoughts.” Only once all the survivors have been permanently rehoused, Dent Coad said, could “the mending” begin. “People’s lives are on hold, they’ve lost businesses, they’ve lost jobs, relationships have broken up. Children aren’t going to school or are failing at school.”

After investigations have concluded later this year, Grenfell will be demolished and replaced with a memorial to the victims. But for now, its charred edifice endures, a reproachful symbol of a stratified society.

Paint the town red: graffiti in Mile End Park. Credit: George Eaton

On a Friday afternoon in Canary Wharf, it is bonus day. Bankers toast their success raucously with flutes of champagne and tumblers of whiskey (financial sector bonuses last year totalled £15bn). This, to put it mildly, does not seem like propitious territory for Corbyn’s Labour Party.

And yet in the City of London – the world’s pre-eminent financial centre – support for Labour is rising, not falling. Brexit, the greatest act of economic self-harm in postwar history, has subverted the natural sympathy many previously felt for the Conservatives (in London, 60 per cent backed Remain, while in the Cities of London and Westminster seat, 72 per cent did).

“The funny thing is that the City is, in US terms, a Democrat place,” Allen Simpson, the chief operating officer of Labour in the City and Barclays’ public policy director, told me when we met at Boisdale restaurant in Canary Wharf. “Everyone is socially liberal, economically liberal. In some bits of the City that pushes you towards Cameroon Toryism, in others it pushes you towards the centre-left. But if you were in America, everybody here would vote Democrat.”

Simpson, 35, added: “There are CEOs of quite large firms – I can think of two – who are Labour members and who voted for Corbyn. They were maybe at Oxford in the Seventies, had long hair and loved a placard – it’s not new for them. It’s not unusual on the trading floors around here to find people who were at the World Transformed conference [held by the Corbynite group Momentum] in Brighton in 2017, it’s the cool thing to be.”

Labour in the City was founded by former Treasury minister Kitty Ussher in 2012 as an association for party supporters in financial services and related industries. Its membership now stands at 650 (having doubled since Corbyn became leader in 2015). John McDonnell, the shadow chancellor and a self-described Marxist, addressed the group last year. “McDonnell has had some very strong meetings with people in the City, he’s taken very seriously,” Simpson said. He did, however, concede that the City had concerns over the “aggregate impact” of Labour’s planned renationalisation programme and tax increases.

Sadiq Khan, who vowed to be “the most pro-business” London mayor in history, and Keir Starmer, the shadow Brexit secretary and former director of public prosecutions, are perhaps the most highly rated Labour politicians among bankers and hedge fund managers. “If you said to the City, ‘Pick a politician you’d like to be in charge of the Brexit negotiations’ they’d pick Keir Starmer,” Simpson told me. “Because he looks like a professional.”

By contrast, City liberals recoil at May’s hard Brexitism and social conservatism. “That ‘enemies of the people’ stuff is profoundly damaging in London and elsewhere,” said Simpson. “Professionals across the piece, metropolitans across the piece, feel very uncomfortable.”

In the City, many are proud “citizens of the world” (rebuked by May as “citizens of nowhere” in her 2016 Conservative conference speech). Nearly three centuries ago, Voltaire distilled this spirit when he visited London’s Royal Exchange in the 1720s: “There the Jew, the Mahometan and the Christian transact together - as though they all professed the same religion, and give the name of infidel to none but bankrupts.”

The animus felt by George Osborne, the London Evening Standard editor, towards May is not merely personal (the Prime Minister sacked him as chancellor and told him to “get to know the party better”) but profoundly political. May has alienated, perhaps for a generation, the metropolitan liberals whom Osborne and David Cameron intermittently wooed.

In London, 86.3 per cent of employees work in the private sector (the highest level of any UK region). That the Conservatives attract so few of these as voters owes much to their metamorphosis from the party of the Economist to that of the Daily Mail. London is replete with the kind of voters the Tories struggle to attract everywhere: the young (the capital’s average age is 36.5), ethnic minorities (44 per cent of residents are non-white), immigrants (38 per cent were born abroad) and graduates (50 per cent are degree-holders, compared to 38 per cent nationally). The Tories’ London problem is often attributed to the housing crisis. Home ownership in the capital has fallen below 50 per cent (compared to the UK average of 63.5 per cent), the lowest level since the early 1980s, and is forecast to drop to 39.5 per cent by 2025. The average house price is £471,986 (compared to the UK average of £225,000) – 14 times the average London salary of £34,200 – and the typical private renter spends 48 per cent of their income on housing costs. As the Thatcherite dream of a “property-owning democracy” recedes, the Tories are struggling to sell capitalism to those with no capital.

“Housing – it’s the number one issue,” Gareth Bacon, the Tories’ London Assembly leader, replied when I asked why the Conservatives had fared so poorly in the capital. “It’s going to be very difficult to build houses quickly enough to satisfy the demand… The people who are in power inevitably get the blame.” Bacon is open to solutions such as allowing all councils to borrow to build (“I can see a lot of good in that. Local authorities do know their areas best.”)

But Ryan Shorthouse, the 32-year-old head of liberal conservative think tank Bright Blue, warned that the Tories risked misdiagnosing their malady: “It’s a cultural problem that young people have with the Tories, not an economic problem with capitalism,” he told me. “If you look at most social attitudes surveys, there isn’t a clamour for socialism among the young, they’re quite sceptical of state welfare, they’re very entrepreneurial. They believe heavily in individual and personal responsibility.”

The Tories’ greatest defect, Shorthouse said, was “Theresa May’s post-liberalism”: “There’s a feeling, which has got worse, that the party is not very socially liberal, it’s not modern, it’s quite sceptical of immigration, it’s sceptical of human rights.” For the Conservatives, retoxified by Brexit, the problem may be less a lack of homes than a lack of heart.

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In 2016 Gavin Barwell, then the Tory MP for Croydon Central, wrote a book entitled How to Win a Marginal Seat. The following year, he discovered how to lose one. Barwell, who is now Theresa May’s chief of staff, was defeated by Labour’s Sarah Jones, who achieved a majority of 5,652.

London’s “blue suburbs” once offered the Tories a path to victory in the capital. Boris Johnson’s “doughnut strategy” consisted of targeting the city’s outer boroughs – older, richer, whiter – in preference to its inner regions. But the boundaries of Johnson’s former imperium have been blurred. Over the last 15 years, outer London has become dramatically younger, poorer and more diverse. Between 2001 and 2011 (the last census), the white population fell from 74 per cent to 61 per cent and the number of private renters doubled to 21 per cent. Outer London now accounts for 58 per cent of impoverished Londoners, up from 50 per cent.

Liberal graduates priced out of Labour boroughs such as Lambeth and Southwark now migrate to Croydon or Bromley. Though their postcodes change, their politics do not. The reddening of the suburbs means Labour aspires to defeat Boris Johnson in Uxbridge (where the Tory majority fell from 10,695 to 5,034 in 2017) and Iain Duncan Smith in Chingford (where it fell from 8,386 to 2,438).

When I met Sarah Jones, 45, at Boxpark in Croydon, a recently opened pop-up mall, she attributed her victory to the Tories’ remorseless spending cuts: “In 2015, people kind of cared about NHS funding, but they didn’t have lived experience of the cuts. By 2017, they did.” Parents were stunned, she said, to find austerity-stricken schools asking them to provide paper and other teaching essentials (books, stationery).

Jones credited Jeremy Corbyn with animating previously inert young voters. “In 2015 a young person would answer the door and say ‘I’ll get my parents’ and we’d say ‘No, we want to talk to you’, but they weren’t interested. In 2017, virtually every young person told me, ‘I’m voting for Jeremy Corbyn.’”

Like other marginal seat candidates, Jones was bolstered by Momentum’s activist army. “We had thousands of people here [in Croydon Central] all the time.” The Corbynite movement, which was founded in October 2015 by veteran Bennite Jon Lansman, now has 40,000 members, 5,800 of whom live in London. It has surpassed the Green Party (39,000) and, on current trends, will overtake the Tories (estimated to have 70,000 members) in June 2022.

At 11am on the morning of 14 April, I watched Momentum marshall its forces in Wandsworth – the Tory borough that served as a laboratory for Thatcherism (selling off council homes, outsourcing services, cutting taxes). Even allowing for the spring sunshine, there was something faintly surreal about the sight of hundreds of activists starting their weekend in a Clapham car park. Young people, it was said before Corbyn’s election as Labour leader, didn’t join political parties.

In canvassing sessions, a traditional refrain was heard from voters: “It’s time for a change.” Leaflets were emblazoned with pictures of local boy Sadiq Khan, who became a Wandsworth councillor at the age of 23 in 1994 and still lives in Tooting. The borough was 75 per cent for Remain in the EU referendum; the 25,000 council homes sold off with the intention of creating Tory hegemony are increasingly occupied by Labour-voting private renters.

I later caught up with Momentum chair Jon Lansman in local pub The Falcon. Lansman, 60, who co-ordinated Tony Benn’s 1981 deputy leadership campaign, is enjoying a late renaissance and was recently elected to Labour’s ruling National Executive Committee. “We’re winning votes both from people who are suffering terrible poverty and inequality and from people who are relatively prosperous,” Lansman told me. “Middle-class people who live in their own homes, and would have expected their children to do so, are finding them leaving university with no prospect of decent housing.”

Yet the last month has been far from an unalloyed success for Labour. More than a thousand people, including leading Jewish organisations, demonstrated outside parliament on 26 March in protest at the party’s lax approach to anti-Semitism. Despite its Corbynite heritage, Momentum has been critical of the Labour leader (who apologised belatedly for defending an anti-Semitic mural – “I sincerely regret that I did not look more closely at the image”). “We have been too slow to recognise the extent of [anti-Semitism] and deal with it sufficiently urgently,” Lansman, who worked on an Israeli kibbutz when he was 16, told me. “We’ve got to have a programme of education and training.”

Does he believe relations between Labour and London’s Jewish community can be repaired? “I don’t think it will happen overnight, people want to see action, not words… But we’re all committed to it.”

When I met Tulip Siddiq, the Labour MP for Hampstead and Kilburn, at the Jewish Community Centre in Finchley (where she holds her surgeries), she lamented: “The Jewish community has really stuck up for me. I do feel devastated that I don’t feel like I’ve been able to do the same for them because of the party.”

Siddiq, whose majority rose from 1,138 to 15,560 in 2017, added: “Lifelong Labour members have come to me and said, ‘Look, I’ll stay with my membership because I believe in the party. But for the first time, I’m not coming campaigning. I don’t feel I can knock on doors and advocate for the party.’”

In Barnet, where 15.2 per cent of the population is Jewish (the highest of any UK local authority), the Tories hope, and Labour fears, that anti-Semitism could yet deny the opposition victory in its most winnable target. The Conservatives have also sought to exploit Labour’s internal divisions. In January 2018, long-standing Haringey council leader Claire Kober resigned, citing “bullying” and “sexism” by Corbyn supporters. The north London borough’s £2bn housing redevelopment scheme had earlier been halted at the request of Labour’s National Executive Committee, following warnings of “social cleansing” by Momentum activists and other critics.

“My strong feeling is that a policy that was not working for the majority of the community was rejected by that community,” Laura Parker, Momentum’s national co-ordinator, told me. “This is a movement which started pre-Momentum, there are people who have been arguing for years that housing redevelopment has to be done in a different way – we didn’t invent that.”

The Conservatives have managed expectations shrewdly: the retention of any of their eight London councils will be presented as a triumph. Yet some are already searching for potential upsides to defeat. “Perversely, if Labour are successful in this election, it could harm them in the long run,” said Gareth Bacon, the Tories’ London Assembly leader. “I’m old enough to remember the ‘loony left’ councils of the 1980s in inner London who ran their boroughs appallingly based on rigid ideology. If we see a return to some of that, people will look and think, ‘We can’t risk this in Downing Street and Whitehall.’ Labour doing well at this election could cause them to do badly at the general election.”

The Tories, however, cannot depend on wishful thinking. At the 2002 Conservative conference, the party’s then chairman warned: “Twice we went to the country unchanged, unrepentant, just plain unattractive. And twice we got slaughtered. Soldiering on to the next election without radical, fundamental change is simply not an option.” Those words belonged to May, and her analysis feels newly relevant. In Scotland, Ruth Davidson’s brand of conservatism – liberal, compassionate, pro-European – has helped the Tories overcome near-extinction. In London, Johnson triumphed in two mayoral contests through a similarly bespoke brand.

“The Conservative Party has recognised that one of the mistakes in the general election was imposing a national message everywhere, regardless of local circumstance,” Bacon told me. “Making the election all about Brexit was not going to be a winning formula. Various figures in the party, including me, have been calling for a London unit, a distinctive London identity, very much along the lines of what we see in Scotland and Wales. Central Office get this. They understand that London is a very important region, we can’t just abandon it. We still have 21 MPs: if we were to lose those we’d be out of office.”

In the last 44 years, the Tories have never won a comfortable parliamentary majority without finishing ahead of Labour in London. Yet some in the party have come close to writing off the capital, advocating a harder-edged conservatism to appeal to Labour voters in the Midlands and the north.

London, whose voting patterns once mirrored those of the UK, now appears a country apart. However, demographers say it also provides a glimpse of Britain’s future: liberal, diverse, highly educated, cosmopolitan. These trends help explain why Labour has already triumphed in such unlikely seats as Portsmouth South, Plymouth Sutton, Leamington Spa and Canterbury.

As they gaze upon Corbyn’s capital, the thought that should haunt the Conservatives is not that they are losing the London of today, but that they are losing the Britain of tomorrow.