It is grim to see the symptoms of new variant malady appearing in Conservative ministers.

It’s a progressive illness. At first, the over-confidence and blind optimism make it difficult to distinguish from other political pathologies. The next stage is subtle also: slogans begin to dictate policy, rather than vice versa. Then political empathy degrades, leaving those afflicted unable to recognise that those on the other side of the negotiating table are also politicians, with their own coalitions to hold together and supporters to satisfy.

Conversely, defences against cognitive dissonance – the ability to cope with two or more contradictory facts – become sharper. Comically, this enables ministers to simultaneously proclaim that 58 Brexit Impact Assessments both exist in excruciating detail, and don’t. Tragically, ministers also assume that all the benefits of the single market can be gained but with none of the obligations. Thankfully, we haven’t yet reached the final “birdman” stage of the disease. At this point, full delusion sets in and victims stand on the edge of the cliff, convinced that as they jump their wings will grow. Often, victims pull down innocent bystanders with them.

UK news in pictures Show all 50 1 /50 UK news in pictures UK news in pictures 18 September 2020 A model presents a creation during the Bora Aksu catwalk show at London Fashion Week 2020 Reuters UK news in pictures 17 September 2020 World kickboxing champion Carl Thomas during his attempt to run a marathon while pulling a plane at Elvington Airfield near York. 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The orphaned pair, who have been living in a temporary home in Belgium since they were found abandoned and alone in a snowdrift in the Albanian mountains, will be acclimatised to their new life in the country before moving to a permanent home PA UK news in pictures 17 August A level students celebrate outside the Department for Education in London after it was confirmed that candidates in England will be given grades estimated by their teachers, rather than by an algorithm. The government U-turn comes just days after Education Secretary Gavin Williamson vowed there would be "no U-turn, no change. PA UK news in pictures 16 August Wasp players take a knee as Northampton Saints stand prior to kick-off in their Premiership match at Franklin's Gardens PA UK news in pictures 15 August Piper Colour Sergeant Lil Bahadur Gurung attends the VJ Day National Remembrance event, held at the National Memorial Arboretum in Staffordshire, Britain Reuters UK news in pictures 14 August People including students hold placards on Whitehall outside Downing Street as they protest against the downgrading of A-level results. The government faced criticism after education officials downgraded more than a third of pupils' final grades in a system devised after the coronavirus pandemic led to cancelled exams yes AFP via Getty UK news in pictures 13 August Benita Stipp (centre) and Mimi Ferguson (left) react as students at Norwich School receive their A-Level results PA UK news in pictures 12 August 2020 A train derailment near Stonehaven has left three people dead. Driver Brett McCullough, conductor Donald Dinnie, and a passenger were killed when the 6.38am Aberdeen to Glasgow Queen Street service crashed amid heavy rain and flooding BBC UK news in pictures 11 August 2020 A woman hydrates in the sun after open water swimming at the West Reservoir Centre in north London Tolga Akmen/AFP via Getty UK news in pictures 10 August 2020 Prime Minister Boris Johnson takes part in an archery session as he visits Premier Education Summer Camp at Sacred Heart of Mary Girls' in Upminster Reuters UK news in pictures 9 August 2020 People cycle through Cambridge as the heatwave continues in Britain EPA UK news in pictures 8 August 2020 Healthcare workers take part in a protest in London over pay conditions in the NHS Getty UK news in pictures 7 August 2020 Emergency services make their way along the seafront on Bournemouth beach in Dorset on one of the hottest days of the year PA UK news in pictures 6 August 2020 Alison Murphy poses for a picture by husband Peter as she walks through a field of sunflowers in Altrincham, Cheshire PA UK news in pictures 5 August 2020 Pakistan's Abid Ali being bowled by England's Jofra Archer during day one of the First Test match at the Emirates Old Trafford, Manchester PA UK news in pictures 4 August 2020 The 'Timbuktu tumblers' from Kenya perform their balancing act on the Southsea waterfront as Zippos Circus reopens in Portsmouth Rex UK news in pictures 3 August 2020 Pelicans interact with a visitor in St James's Park in London PA UK news in pictures 2 August 2020 Lewis Hamilton drives with a puncture towards the finish line to win the Formula One British Grand Prix at Silverstone POOL/AFP via Getty UK news in pictures 1 August 2020 Arsenal's Pierre-Emerick Aubameyang celebrates with the trophy and teammates after winning the FA Cup, as play resumes behind closed doors following the outbreak of the coronavirus disease Pool via Reuters UK news in pictures 31 July 2020 People enjoy the sunny weather at a Bournemouth Beach Reuters

My party, Labour, has inoculated itself temporarily with a decision to argue for a transition period in which the UK stays in the single market and customs union. This is the least damaging form of Brexit for the economy, and may be the only way of avoiding a hard border in Northern Ireland. However, we are not immune. On our current trajectory it is possible that either in opposition or in government Labour will support the hard Brexit of leaving the single market and customs union.

Labour has done great work in forcing ministers to publish those 58 impact assessments. But when we read them, what should we do?

Reading the impact of Brexit on individual sectors of the economy will be like colouring in one piece of a painting-by-numbers: we already know what the Government’s overall assessment looks like. This has been leaked to the Independent and quoted, and is essentially based on the original Treasury analysis. They estimated that the long term economic loss of leaving the single market would be between 6.2 to 7.5 per cent of GDP every year. This leads to lower tax receipts of £36bn to £45bn each year, equivalent to about a third of the NHS England budget. Independent forecasts paint a similar picture: the further we travel from the single market, the greater the economic loss.

Campaigners also hope to force the release of the Treasury’s analysis reportedly showing that the losses caused by leaving the single market and customs union will be far greater than the gains from new free trade agreements. Analysis from the National Institute for Economic and Social Research estimates that new trade deals with Brazil, Russia, Indonesia, India, China, South Africa, the US, Canada, Australia and New Zealand would boost UK trade by five per cent, while replacing the single market and customs union with a new free trade agreement with the EU would reduce British trade by 22 per cent.

David Davis: "We will look at" plans for Brits to keep EU citizenship after Brexit

Once Labour reads these assessments we should change course and commit to staying in the single market and customs union. If we are in opposition, it is only through this clear policy that we will attract enough Tory rebels to prevent a hard Brexit. If we are in government, then we will own the consequences of the decision to leave.

One consequence is the risk of more years of austerity. In 2013 the NHS estimated it needed another £30bn per year by 2021 to cope with an ageing population. If we leave the single market, we risk forgoing tax receipts of £36bn. That’s quite a gap to fill.

None of this is easy. Labour has a small number of influential people who worry, unnecessarily, about EU state aid rules and nationalisation. It is highly implausible that these rules will prevent any policy proposed in the 2017 manifesto. Jeremy Corbyn should urgently commission legal advice to satisfy himself on this matter. A bigger problem for the party is migration. Many MPs see the Brexit vote as ruling out freedom of movement. While there are some useful suggestions of additional marginal controls on immigration that can be implemented while remaining within the single market, fundamentally Labour must make a choice between ending freedom of movement and saving the British economy. No one has yet split the single-market-freedom-of-movement atom.

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If this government falls and Labour takes the reins, our Ministers will be receiving the same advice that Conservative ministers are currently ignoring. Unless we change course it is Labour ministers who will take Parliamentary questions on the economic impact of hard Brexit and struggle to avoid delusional statements. It would be a tragedy if this Brexit malady afflicting the Conservatives spread to the Opposition benches. My hope is that by committing to remaining in the single market and customs union, we can prevent this new disease from jumping the barrier separating our party from the delusional Brexiteers sitting on the Conservative front bench.