At the end of this week, it will become clear whether the prime minister will secure a new withdrawal agreement with the EU or not. All the signs are that he will not, so minds are focused on whether he will comply with the spirit and letter of the Benn Act, named after its chief sponsor, Commons Brexit Select Committee chair, Hilary Benn.

The Act requires the PM to request an extension of Article 50 in the event of no deal on or by 19 October. Last week, the government submitted documents to the Court of Session in Scotland, in litigation relating to the Benn Act, in which it confirmed the PM will send a letter to the EU Council asking for an extension to Article 50 as required by the Act despite Boris Johnson’s repeated insistence that he will never do so.

In the event the PM does not ask for an extension, as an insurance policy there is talk of a motion of no confidence in the government being tabled, Johnson being removed and replaced with a PM of a caretaker government (or “government of national unity” as some call it) who can be relied on to send that letter before exit day, currently scheduled for 31 October. It is not clear who the Queen would turn to and when to form such an alternative government – it may well be the leader of the opposition, but does he have the numbers?

Various senior Labour figures, from John McDonnell to Diane Abbott, have been peddling the myth that were the Liberal Democrats to support a Jeremy Corbyn premiership in these circumstances, Corbyn could form a government. This is nonsense and they know it. Whatever claim he may have to lead such a government, and notwithstanding whatever we do, the Labour leader does not have the numbers in parliament to form such an administration. Former US president Lyndon B Johnson famously said that the first rule of politics was being able to count – senior Labour folk are struggling to do so here.

If all Labour MPs, the SNP, Plaid Cymru and the Green Party’s Caroline Lucas were to support a Corbyn caretaker administration, he would still be at least 32 votes short of a majority regardless of what the Lib Dems do. This is because all but one of the 20 anti-no deal Tory rebels have made it absolutely clear they won’t support him under any circumstances – the same is true of a number of independent MPs and others. The detailed figures on the parliamentary arithmetic are here.

So, if not Corbyn, who? All other opposition parties have said we will be pragmatic on who might lead a caretaker administration. Lib Dem leader Jo Swinson, far from insisting she get a chance to form such an administration (and although she leads a party that got 20 per cent of the vote in the last nationwide elections this year to Labour’s 14 per cent and the Tories 7 per cent), has put forward the names of the Mother and Father of the House of Commons, Labour’s Harriet Harman and the Tories’ Ken Clarke, as more neutral figures.

The Tory rebels – whose support would be needed – are not insisting a Tory MP leads a caretaker government and recognises the need to compromise too. SNP leader Nicola Sturgeon has said she is “open minded” on who it should be. In fact, every other opposition party has said publicly that it is prepared to consider supporting an MP of another party to lead such a project. The only party insisting its “my way or the highway” and which has refused to compromise in any way at all on this issue is the Labour Party. McDonnell let the cat out of the bag over the weekend in an interview with Italian newspaper, la Repubblica. He told them: “We’ll never accept an interim government without Jeremy Corbyn as PM.” McDonnell told the newspaper this was the case even if a caretaker government under a different PM was the only way to stop a no-deal Brexit.

Privately, Labour’s leadership concedes that there are no circumstances where Corbyn could command a majority in this parliament, even for a temporary period, and yet they persist on insisting that only he should lead such a government. The effect of Labour’s intransigence and partisanship is to kibosh any chance a caretaker government could act as an insurance mechanism to stop no deal. The final question is therefore for Corbyn: why, when everyone else has, won’t you compromise if it could stop no deal?

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Actions speak louder than words. The Liberal Democrats have always opposed Brexit and were arguing to stop it when it was deeply unfashionable to do so immediately after the 2016 result. So keen was the Labour Leader for the UK to head for the exit door that he was insisting Article 50 be invoked the day after the 2016 referendum result. This is because neither Corbyn nor McDonnell cares much for EU membership, beyond the needs of political management inside and outside the Labour Party. That is what has driven their contorted position on Brexit since Corbyn became leader in 2015, not any desire to Remain.

They both spent decades campaigning for us to leave the EU. Indeed, the Labour leadership’s campaigning for Remain in 2016 was lukewarm because Corbyn didn’t believe in the cause – he never has. They have carried on in this fashion since the Brexit vote. On the one hand, when I put down an amendment to the 2017 Queen’s Speech to keep the UK in the single market and customs union – an amendment which also specifically called on ministers to “rule out withdrawal from the EU without a deal” – Corbyn whipped his MPs to vote against it and sacked three Labour frontbenchers for supporting it. On the other hand, when 25 Labour MPs, including eight shadow ministers, failed to support legal measures in the commons to stop a no-deal Brexit in January of this year, not one frontbencher lost their job. In short, remaining in the EU is not a cause for which Corbyn is prepared to put the national interest before his party political interests. That’s what’s really going on here – the rest is noise.