"MAKE Brexit Work” sounds like one of Boris Johnson's test slogans for the next General Election. In fact it is now Labour's headline policy following Sir Keir Starmer's remarkable conversion to the Brexit cause. He unveiled the new slogan in a speech to the Centre for European Reform.

It's an unexpected gift to Nicola Sturgeon, whose latest Indyref plan was getting heavy weather from sceptics in and out of the SNP. Suddenly she leads the only serious pro-European party in Scotland, apart from the Greens and the LibDems. Getting 50 per cent of the vote in the 2024 General Election looks easier now that the SNP is the obvious destination for the 62 per cent of Scots who voted to remain in the EU.

Sir Keir has also declared war on his own party, many of whom are as pro-European as the SNP. The London Mayor, Sadiq Khan, has already broken ranks and rejected his leader's policy switch. Others in the Labour Party are bewildered. It seems only yesterday that Sir Keir was arm in arm with Nicola Sturgeon leading those raucous anti-Brexit marches and demanding a repeat referendum.

Wasn't Brexit all about “disaster capitalists” conning the white working class with “nationalist populism” the better to instal a “proto-fascist state”? Just read the books and tweets of Labour-supporting authors like the former BBC Newnsight journo, Paul Mason, and the #FBPE philosopher AC Grayling. To many pro-Labour intellectuals, Brexit is simply Britain's version of the Trump apocalypse.

Remainism wasn't just the calling card of the Labour left. Uniquely, opposition to Brexit united left and right in the Labour Party. The king over the water, Tony Blair, was the inspiration for the pro-European Labour folk like Hilary Benn, the architect of the Benn Act during the great parliamentary struggles in 2018-19. Their passionate followers won't abandon faith in free movement over night.

And what about all those Labour luvvies who signed petitions against hard Brexit – Steve Coogan, Hugh Grant, St Bob Geldof – not to mention national treasure Emma Thompson, who said Brexit was “mad” and departed for Italy (briefly)?

Well, yes, that kind of answers the question of why Sir Keir has changed his spots. The outpouring of grief by the Remain elites and their contempt for Brexit voters, did not go down well with Labour's many working class supporters, hence the Red Wall crumbling to the Tories in the 2019 General Election. Sir Keir had no alternative but to come to terms with Brexit, since it clearly isn't going away.

However, I remain mystified at both the timing and the nature of the Labour leader's conversion to what used to be called "hard" Brexit. The lesson in politics is supposed to be that when your enemy is making mistakes, don't get in the way. Boris Johnson's grip on those northern working class votes is looking a lot less secure following recent by-elections.

Sir Keir could have accepted the reality of Brexit – indeed he already has many times – without ruling out any kind of accommodation with the EU single market . Yet in his speech on Monday night he made absolutely clear that Labour will have nothing to do with any EU institutions. “We will not be joining the single market. We will not be joining the customs union...we will not return to freedom of movement”. Sir Keir is for pure, clear Brexit.

Read more: Remainers aren't to blame for Brexit, but they made it a lot harder

If things were going reasonably well this capitulation to hard Brexit might have made sense, but even Brexiters are now saying that leaving the EU single market has damaged the economy beyond immediate repair. It is beyond dispute that the barriers to trade erected by the EU since 2016 have wrecked exports to the UK's largest market. As predicted, the EU has turned its full protectionist guns on Brexit Britain.

The Centre for European Reform, the think tank to which Sir Keir unveiled his new slogan on Monday, has done a post-pandemic comparison with similar economies that remained in the EU. It showed that the British economy was five per cent smaller than it would have been had we remained in the EU single market. That means Britain is £31 billion a year poorer.

The main problem with Sir Keir's attempt to Make Brexit Work is that he will have to confront the unpleasant reality that the EU is determined to make it NOT work – certainly in Northern Ireland. Sir Keir claims to have a solution to the problems caused by the Northern Ireland Protocol, removing the “giant fatberg of red tape and bureaucracy” as he put it, which has led to a border emerging in the Irish Sea.

Brussels has imposed that fatberg, not the UK Government. It says the endless checks and rules are necessary to protect the European single market. This has meant, famously, that Sainsbury's meat products, like sausages, could not go on sale in part of the UK because the province was remaining, effectively, under Brussels’ regulations.

If Sir Keir has a solution to this, then I'm sure the UK Government would love to hear it. He seems to think that because he is an “honest broker” the constitutional issues can be magicked away. But this about a lot more than just harmonising veterinary standards.

Britain says goods destined for Northern Ireland must be allowed to flow there without checks and impediments. This is unacceptable to Brussels because it risks goods being for sale in a part of the EU single market, Northern Ireland, which are not subject to the rules of the European Court of Justice. Sir Keir is now saying, in terms, that the court cannot continue to have that jurisdiction whatever the protocol says.

Sir Keir cannot condone a border appearing within the sovereign UK. Nor can he argue, like Sadiq Khan and some Tory MPs, that Britain should go back into regulatory alignment with the EU single market.

He is now effectively aligned with Boris Johnson in the Brexit camp. That is an uncomfortable place to be at the best of times. It risks splitting the Labour Party and handing Scotland to the SNP.

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