Even a broad church must be bound by a common faith. Labour’s founding purpose was to secure parliamentary representation for organised labour, and to use that power to advance the collective conditions of working people. Sure, there were some interpretations of that historic mission that were more radical than others. Some believed it meant humanising the existing system, others replacing it altogether. (Even New Labour, which struck an accommodation with Thatcherism, invested in public services, the minimum wage and tax credits to improve the lot of millions of working-class people.)

A striking exception in Labour history was the premiership of Ramsay MacDonald who, in 1931, attempted to slash public spending and unemployment benefits. When his own party refused to sanction such a remorseless assault on the British working class, MacDonald formed a coalition with the Conservatives, provoking swift expulsion from Labour and leaving his legacy in the history books a byword for betrayal.

Workers' rights, environmental and social protections: they will be tossed on a bonfire

Which brings me on to those Labour MPs minded to vote for a Tory Brexit deal, including one negotiated by Boris Johnson to suit the ideological fantasies of his party’s right. Theresa May was forced to commit to maintain a level playing field with the EU’s workers’ rights and environmental and social policies: in other words, they would remain the baseline, with Britain only able to improve on them. Not so under Johnson: such protections will be tossed on a bonfire, triggering a race to the bottom in hard-won rights.

When Angela Merkel talks of a post-Brexit Britain as a potential competitor, she doesn’t mean because Germany fears a British lion flourishing after being liberated from its cage, but rather of being undercut by inferior rights, protections and wages. This paves the way, of course, for a deal with Donald Trump’s US, where social guarantees are far weaker than in the UK. The Tories have dismissed the president’s original call for the NHS to be on the negotiating table, and yet the only consistency in the Johnson administration is untrustworthiness. Then there’s the peril facing what remains of British industry: according to a letter signed by five sectors – ranging from automotive to pharmaceutical – Johnson’s planned dealignment with EU regulations threatens “huge new costs” and disruption.

A Labour MP who votes for a Johnson Brexit deal is voting to slash the living standards, rights and jobs of the very people their party exists to represent and champion. The Tory right always saw Brexit as an opportunity to recast society in their hyper-Thatcherite, dystopian image, where the rich and big business pay little tax, where regulations are eliminated, where workers can be hired and fired on a whim, and where the frontiers of the state can be rolled away.

A Johnson deal is class war from above, a Tory heist, a scalpel taken to the country’s social fabric. And if a Labour MP votes for such a deal, their betrayal of the most basic reasons their party even exists means they should no longer be a Labour MP, that the party’s leadership should withdraw the whip, or the national executive committee should deselect them.

There are 19 Labour backbenchers who have written to the EU urging a deal to be negotiated with the Tories that they can vote for. How many would seriously vote for Johnson’s deal is open to question. But if a Labour MP voted for, say, a Conservative budget or Queen’s speech, they would expect to be evicted from the parliamentary party. This is worse by some magnitude.

Such MPs would hand Johnson an electoral boost before an election that could decide Britain’s political direction for a generation. Even a moderate rebellion will grant the Lib Dems a political gift, allowing them to disingenuously claim Labour shares responsibility, advancing their mission to split the anti-Tory vote.

While it is true that Labour MPs representing leave seats feel that a remain-dominated left does not understand or empathise with their plight, they don’t tend to have doors slammed in their faces by angry remainers insisting that Labour more enthusiastically embrace their cause. It is much more likely to be leave voters who engage in that type of behaviour on the doorstep, demanding to know why the referendum result has not been implemented. The MPs too are provoked by Labour leave voters questioning their “Labour values” .

Yet they must know that those leave voters did not actually seek to trash their rights at work, to destroy their employment or to attack environmental protections when they voted to leave the EU in 2016. If they wanted Thatcherism on speed, they would not be voting for the Labour party. What do claims of “Labour values” amount to other than defensive platitudes if a member of a self-described democratic socialist party is willing to vote for ideological schemes originating in the writings of the uber free-market economist Friedrich Hayek?

Whether a Labour parliamentarian hails from the left or right flank of their party is irrelevant. If voting for this cataclysmic deal is not incompatible with party membership, then what is? And what future awaits these MPs when the horror show they’ve signed off on unfolds? The Tory Brexiteers will never respect them, their movement will reject them, and their voters will one day blame them. Ending their political career may be distressing enough, but they may soon find that losing their political dignity is even more haunting.

• Owen Jones is a Guardian columnist