John McDonnell is stumped for words. He sits in silence, the only sound the hiss of a coffee machine at the back of the café. The pause drags on while the man who could soon be in charge of the British economy tries to answer the question: “Who are your business heroes?”

McDonnell lists his hobby in Who’s Who as “fermenting (sic) the downfall of capitalism”. As a previously self-proclaimed Marxist — and past admirer of Venezuela’s socialist state — he is perhaps the most leftwing person ever to hold the role of shadow chancellor.

He was recorded at one meeting in 2011 encouraging unhappy workers to spit in their bosses’ tea. Ken Livingstone, the leftwing former mayor of London, sacked him in the 1980s for being too radical. For certain business leaders, the idea of McDonnell in the Treasury is starting to eclipse Brexit as a pressing political concern.

Back in the café, a reply comes at last. “That’s a good question, really,” he says, slowly. There is another pause. His press adviser offers some ideas: “You like Dale Vince?” he says, referring to the renewable energy entrepreneur. There is another pause.

Eventually the words tumble out: “There’ll be creative business leaders but actually, when it comes down to it, they can’t do anything unless they’re part of a collective,” says McDonnell. “Unless they’ve got that wealth creator, that engineer and that work person, that skilled person at the bench to fulfil that idea . . . they’re nothing.”

John McDonnell in a café in his Hayes & Harlington constituency last month © Cian Oba-Smith

Of late, McDonnell has seemed to cut a more pragmatic figure, sipping tea with asset managers and sharing football banter with Mark Carney, governor of the Bank of England. The day before our interview he was meeting business leaders in Birmingham. The previous week he was at the London Chamber of Commerce: “It was packed out . . . I’ve been saying to them, ‘Look, there’s no tricks up our sleeves. I want these ideas tested almost to destruction.’ ” He even attended this year’s Davos, albeit to offer a warning about the “political and social avalanche” set to hit the world elite.

So who is the real McDonnell? Diane Abbott, an old ally and Labour MP, observed last year: “John McDonnell has done his best to transform himself into a friendly, bank-manager-type figure, which, if you know John McDonnell as well as I do, is . . . interesting.”

Today, the MP is hosting the FT in his constituency. Hayes & Harlington, a gritty west London seat divided by the M4, is home to a high immigrant population and Heathrow airport. McDonnell takes a close interest in deportation claims, welfare battles and — increasingly — housing issues. “We’ve got people living in the most appalling conditions, overcrowding, people sleeping in the streets and on the canal, in the parks; we also have the phenomenon of beds in sheds.”

Until May 2015, John McDonnell, 66, was a historical footnote. The silver-haired leftwinger tried to stand as Labour leader in 2007, promising to break the stranglehold of the “New Labour” elite, but failed to get on the ballot. In 2010 he tried — and failed — again. By the general election of 2015, he had suffered a heart attack and seemed set for oblivion. “I was going to happily drift into retirement and sit at the back of halls complaining,” he says. “Then Jeremy decided to have a run.”

Today, a McDonnell chancellorship is a realistic possibility. Labour is neck and neck with the Tories in the polls and secured 40 per cent of the vote last summer — just two points behind Theresa May’s party. McDonnell believes he is on the brink of making history, should the government collapse because of Brexit.

“Our objectives are socialist. That means an irreversible shift in the balance of power and wealth in favour of working people,” he explains. “When we go into government, everyone will be in government.” In keeping with these turbulent political times, he is currently reading Imperium, the Robert Harris book about Cicero and the Roman Republic. Asked whom he empathises with in it, he replies: “The poor guy Cicero employs to write all this up.”

McDonnell was born in Liverpool on September 8 1951. His father was a union-activist docker turned bus driver and his mother worked as a cleaner and then at BHS behind the biscuit counter. (“We had broken biscuits on a regular basis.”) They lived in a house with an outdoor loo and a tin bath: “I know it sounds a bit Monty Python . . . Sociological studies now say it was one of the worst slums in Europe but we used to just call it home.”

The family subsequently moved to Great Yarmouth, Norfolk, and McDonnell, a former altar boy, was sent to Ipswich to train as a priest. “I discovered two things: girlfriends, so celibacy would have been a problem. The second was politics . . . I developed a different belief system.”

After leaving school at 17, he held a series of unskilled jobs, including working 12-hour shifts making Philips TVs. He took A-levels at night school and studied politics at Brunel University and then Birkbeck. He and his first wife became “house parents” for a children’s home in Hayes. “Some of them had been through terrible experiences,” he recalls. “We ran it like our own family, wherever we went they went with us. We’d go to see my Mum with eight to 10 kids in my Morris Traveller.” He remarried in 1995, and has three children and five grandchildren.

McDonnell was also working as a union official before being elected to the Greater London Council, becoming chair of finance at just 29. At the annual GLC pantomime, he played the cat to Livingstone’s Dick Whittington. Another year, he played a policeman alongside Paul Boateng dressed as a judge in a basque. “I’m just glad there isn’t a film of it.”

Jeremy skates round the country with almost karmic serenity winning admirers . . . John does most of the heavy lifting

Four years later he was ousted over the “rate-capping” of council tax. At the heart of the row was a claim that McDonnell had pretended the Tory government’s rate cap would require £140m of cuts. Livingstone told him: “We’re going to look like the biggest f***ing liars since Goebbels.” McDonnell labelled Livingstone “a Kinnock” — shorthand for a sell-out. The two men didn’t speak for a decade, according to Livingstone. Cast out of City Hall, McDonnell co-edited a controversial leftwing newspaper.

After becoming an MP in 1997, McDonnell busied himself in various ways: chairing meetings of the anti-Heathrow expansion group, setting up an umbrella group of leftwing unions and leading a campaign against the construction industry, which had been blacklisting workers for their political views and union activities, leading to a £75m settlement. No one doubts his work ethic. Dave Smith, head of the Blacklist Support Group, says he would often encounter McDonnell at a picket at 6.30am: “When no one else was prepared to talk to us he was there . . . representing working people fighting for justice.”

But it was the election of Jeremy Corbyn as Labour leader in 2015 that transformed the fortunes of his closest ally. The pair first met in London in the 1970s. “We hit it off straight away,” recalls McDonnell. They would head to a demo then go to Gabi’s, a café off Trafalgar Square: “I’d have a salt-beef sandwich and he’d have a vegetarian dish.”

With Jeremy Corbyn at Labour’s 2015 conference © Getty

McDonnell and Corbyn are uncannily like-minded: both opposed the Iraq war, were early critics of PFI schemes, were sceptical about the EU and fought for various unpopular causes at fringe events. “We’ve never fallen out, we’ve never had an argument . . . we’ll disagree on tactics but we usually arrive at a consensus,” says McDonnell. They voted together against the Labour party leadership hundreds of times.

When Corbyn became leader, he ignored pleas from some trade union allies to put a less divisive figure in the pivotal role of shadow chancellor. Paul Kenny, former head of the GMB, who fell out with McDonnell over an industrial dispute, is forthright: “My experience with him in recent years is that he’s untrustworthy.”

Under Corbyn, Labour has shifted decisively to the left, flush with 400,000 new members inspired by his uncompromising commitment to equality, as well as specific pledges such as scrapping tuition fees and more money for the NHS. Yet while Corbyn is the symbol of the movement, McDonnell plays an arguably more important role in Labour’s new management. While the leader gives speeches to adoring crowds, McDonnell is more of an organiser.

He’s a Jekyll & Hyde character. You never know if you’re seeing a sensible bank manager or a revolutionary Trot

An avid reader, he is inspired by Gramsci, the Italian Marxist who believed socialism would triumph by infiltrating “schools, universities, churches and the media”. “[McDonnell] is the brains behind Labour’s hard-left strategy. I think nobody should underestimate him, he works hard, he is effective, methodical, but he is utterly ruthless,” says Margaret Hodge, a Labour MP who first met him in the 1980s. She compares him to a “Jekyll and Hyde character”, saying: “You never know for sure if you’re seeing a sensible bank manager character or a revolutionary Trot.”

Alan Simpson, a former Labour MP — and friend of both Corbyn and McDonnell — says they are the “Torvill and Dean” of British politics. “Jeremy skates round the country with almost karmic serenity, winning admirers for his openness and integrity,” he says. “John, meanwhile, plans the detail of their programmes and does most of the heavy lifting.”

A less tactful colleague describes Corbyn as a “dosser”: “John is a Stakhanovite, Corbyn is lazy, he’s the bloke who just turns up, while John is the one who reads the papers beforehand.”

Corbyn’s son, Seb, is an adviser to McDonnell, although the MP recoils at any suggestion of nepotism: “Whether or not his name was Corbyn I didn’t care . . . he’s just extremely talented.” McDonnell also bristles at the idea that he is the real strategic organiser. “I’ve heard this rubbish. That’s absolute rubbish because when we do things, we do things together,” he says, tightening his fist and jabbing it forward. “People underestimate Jeremy . . . don’t underestimate his ability for strategic decision-making.”

While McDonnell has a sense of humour, his dour public demeanour — he speaks in flat tones, with a trace of Scouse — is in contrast to Corbyn’s grandfatherly charm. He does not entirely reject the idea he is the Lennon to Corbyn’s McCartney. “He [Corbyn] is one of the most caring people I’ve ever met . . . he doesn’t like conflict but he’s absolutely principled,” he says. “Me, I’m slightly different, more than slightly different . . . I will be more in people’s faces and we’ve complemented one another on that basis.”

The so-called hard man of the left appears a more mellow figure these days, clutching an FT as he meets eminent figures from the CBI, Whitehall or the BBC. When the FT first requested an interview, he texted back to warn that he would “not be very interesting”.

Yet there is a volatile streak: whether hurling Mao’s Little Red Book across the despatch box in 2015 or waving the mace around the Commons in protest at Heathrow’s expansion. In 2014, McDonnell approvingly quoted the idea of Tory MP Esther McVey being lynched, and he once joked about “garrotting” Danny Alexander, a former Lib Dem cabinet minister. “He survived, didn’t he?” McDonnell retorts, when asked.

Challenged on his violent rhetoric, the MP claims he is merely a “street orator”. “I call a spade a shovel, straightforward. If I disagree with someone, I tell them.” He has faced abuse for his views too, he says, for example when he advocated gay-friendly policies at the GLC in the 1980s: “I had my window smashed, milk bottles broken in my children’s sandpit and maggots poured through my letter box.”

More serious are the claims around what Nigel Dodds, a DUP MP, has called “a sinister . . . attachment to violent Irish republicanism”. McDonnell was forced to apologise for saying in 2003 that the IRA should be honoured. He had declared: “It was the bombs and bullets and sacrifice made by the likes of Bobby Sands that brought Britain to the negotiating table.” In 2015 he told the BBC: “If I gave offence — and I clearly have — from the bottom of my heart I apologise. I apologise.”

Yet in his study in Hayes, among old copies of Hansard and files labelled “human rights” and “groceries adjudicator bill”, there is still a plaque dedicated to “H-Block Martyrs 1981”, a reference to the 10 IRA and INLA prisoners who died during the hunger strikes, including Sands. McDonnell denies that he resisted the peace process: “I’ve always honestly and openly said I believe in a united Ireland, but the point was to try and get to a united Ireland without the violence.”

Livingstone, who is currently suspended from the Labour party, recalls: “People like me and Jeremy and John wanted to stand up for the Irish who had been persecuted and mistreated. But you never heard them say, “Oh good, there has been a bomb.”

At the GLC with, from left, Ken Livingstone, Val Wise, Charlie Rossi and Michael Ward (far right), 1982 © Getty

In a 2006 interview, McDonnell said his inspirations were “the fundamental Marxist writers of Marx, Lenin and Trotsky, basically”. When he ran a book club at the Trades Union Congress, members would joke that they had to read the same book every week: Das Kapital.

Rob Marris, a former MP who served in McDonnell’s first Treasury team, says he is only interested in ideas that accord with his world view: “He is inclusive, as long as you don’t cross him.”

Some colleagues believe McDonnell still craves the leadership. That could be eased by a likely change in the rules allowing candidates to stand with fewer MP signatures. But with his health record, the clock is ticking. Indeed, he denies any such ambition: “There’s no hope. Literally no — and I’ve made that clear time and time again.”

Until last summer the Corbyn-McDonnell axis was seen as a sideshow, given the huge Conservative poll lead. But since Labour gained 30 seats in the June election, and Theresa May’s cabinet has become increasingly divided over Brexit, business leaders have been queueing up both to meet the shadow chancellor and express their scepticism about him.

There has been an unlikely mutual courtship dubbed “the tea offensive” by the McDonnell team, with him and Corbyn regularly meeting business groups such as the CBI and FSB. “They need to know where we’re coming from . . . it will be amicable and friendly . . . but we don’t accept any money from bourgeois organisations.” He does, however, make an exception for the owner of the café we are sitting in, who brings over some free baklava.

McDonnell insists that companies are “looking to [Labour] for security” in an uncertain world. His policies span the radical to the mainstream, including the nationalisation of some utilities, higher taxes to fund a more generous welfare state, billions of borrowing for infrastructure and the relocation of most of the Bank of England to Birmingham. He is interested in alternative models of ownership and in copying the Scandinavian model of enhanced profit-sharing for employees. “Are those ideas inspired by Marx? They’re inspired by socialists and, of course, Marx is one of those thinkers, alongside RH Tawney, GDH Cole, William Morris,” he says.

McDonnell has had an open-minded reception from some business leaders who still resent the Tories over Brexit. They want Labour to pressurise the government into a softer Brexit, despite Corbyn and McDonnell’s history of Euroscepticism: it was said in the past that “even one single market is too many markets” for them.

McDonnell was accused of mixed messages in the referendum campaign, in which he used the phrase “Project Fear” to describe the Remain campaign. Reminded of this, his body language becomes tense, his shoulders hunched: “This argument that Jeremy and I didn’t campaign enough: that’s rubbish. We worked bloody hard,” he says. “But it was like a by-election where everyone’s grievances piled into that one vote.”

McDonnell voted Remain and claims to have been gutted by the result. Either way, he has been cautious about Labour shifting away from its current ambiguity over Brexit: “You can alienate people, it’s like walking a tightrope.”

At an anti-Heathrow-expansion protest, October 2015 © Getty

McDonnell believes there is other potential common ground with business, such as extra money for skills and infrastructure. “Where we disagree is on how to get there,” says one business leader who has met the shadow chancellor.

The manifesto promised £48bn of additional annual spending, funded by higher taxes on business and the rich. There would be an extra £250bn of borrowing over 10 years — but only for infrastructure.

In the autumn, the CBI warned that one Labour policy — nationalising railways, water companies and the energy grid — would “send investors running for the hills”. Parliament would decide the level of compensation for the nationalisations, funded by the issuance of new gilts. Terry Scuoler, until recently head of the Engineering Employers’ Federation, has warned that McDonnell in the Treasury would be a “nightmarish” prospect for business. “There are a number of policy initiatives which are likely to be the thin end of a hardline socialist wedge.”

The prospect of much higher taxes and borrowing is anathema to many in the business community, and a double threat to those already concerned about Brexit. One executive in the City says colleagues are divided on how to deal with Labour: “There’s a school of thought that we need to engage and try to influence them, but then others say you can’t put lipstick on a pig,” he says.

Labour suggests its election manifesto from June was mainstream: “Corporation tax would be lower than under Tony Blair. Income tax would be lower than under Thatcher,” says one front-bencher, who contrasts it with the “crackers” manifesto of 1983, which proposed a ban on imports of foreign cars. Meanwhile, business leaders hope that revolutionary ideas could be curbed by more moderate forces within Labour — or by coalition with the SNP or Lib Dems.

The main concern in the business community is whether the manifesto is the limit of McDonnell’s ambitions or just an appetiser for a more radical programme once elected.

John McDonnell in his Hayes & Harlington constituency last month © Cian Oba-Smith

A centrist Labour MP predicts the shadow chancellor will “go for broke” if he ever gets into power. “There are a lot of people who will never, ever trust him . . . he’s very, very authoritarian.”

One person who has worked with McDonnell says people are right to be sceptical: “I think he views business people and centrists as ‘useful idiots’, to quote Lenin,” he says. But McDonnell insists that remaining Blairites have nothing to fear: “Having a broad church is useful, it stops a situation where the emperor has no clothes.”

Richard Barbrook, a self-styled “cybernetic communist” who advises the shadow chancellor on the digital economy, says McDonnell would like to be more radical than the 2017 manifesto, “but he wants to take other people with him”. McDonnell tries to sound reassuring: “We’ve set out what we’re going to do and that’s it.”

Adopting a bipartisan approach once in office has happened before. As mayor of London from 2000 to 2008, Livingstone surprised critics with his own cross-party stance. “When John becomes chancellor he will have the same problems that I had: you can’t get everything you want through,” he says. “They won’t stand on one manifesto and then go off in a more radical direction.”

Having a broad church is useful. It stops a situation where the emperor has no clothes

And there have been signs of pragmatism from McDonnell. One such was his campaign against the Heathrow third runway, during which he worked with Tory MPs including Zac Goldsmith and Justine Greening. “Although our politics couldn’t be further apart, I’d certainly consider him a friend,” says Goldsmith.

Ironically, some leftwingers question whether McDonnell would be a radical chancellor, asking why, for all his talk about research on inequality by economist Thomas Piketty, there was no wealth tax in the manifesto. “Some of us don’t know any more if he wants a socialist country or just wants to tweak the capitalist system,” says a colleague.

As he tries to recalibrate his public image, McDonnell has been urged to “pace yourself” by doctors. But even as he went to hospital after his heart attack in 2013, he couldn’t drop the politics. “We were talking about consultants’ pay and how anaesthetists aren’t properly represented in Parliament,” he recalls. “I said, ‘Lads, you should concentrate on this.’”

Talking about free markets at Davos earlier this year © World Economic Forum / Greg Beadle

Colleagues say his timetable is increasingly stretched given his constituency duties, his role as shadow chancellor and his party role: he could be 70 by the next election. “There’s a lot of strain on him for a bloke who’s had heart problems,” says one MP. “You never, ever see him relaxed.”

The little time off he has is spent with his favourite authors — who include Charles Dickens, particularly Bleak House — and watching football. Roy Bentham, a blacklisted construction worker from Liverpool, says McDonnell sometimes visits Anfield: “People come over all the time and shake his hand and chant his name in the pub . . . he has been through the bad times in the Labour party and is now on the cusp of something big.”

McDonnell tries to spend at least one weekend a month in Norfolk. He recently bought a boat, only discovering afterwards that it was called “The Morning Star” (also the name of the socialist newspaper). “I didn’t know the name of the boat when I bought it . . . I thought, no one will believe me.”

This article has been amended to correct the acronym for the Irish National Liberation Army (INLA).

Jim Pickard is the FT’s chief political correspondent

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