“I'm a Gnu. How d'you do? You really ought to know who's wa-who”. So sang the musical comedy duo, Flanders and Swann, referring to the elk-like creature that roams the tundra. Last week the GNU was stalking Westminster, and proving elusive as MPs sought a Government of National Unity, GNU, to halt Brexit.

The obvious person to front such a caretaker government, should Boris fall after a vote of no confidence, is of course the Leader of Her Majesty's Opposition, one Jeremy Corbyn. He leads the largest party, after all, and speaks for the opposition at the Dispatch Box every week. As Nicola Sturgeon made clear, he has the constitutional right to seek to form an alternative administration.

But the Liberal Democrats, and many rebel Tory MPs aren't having it. The thought of Corbyn in Number Ten, even for a long weekend, is just de trop. The LibDems and Labour right-wingers wanted someone else, anyone else to lead the GNU: an elder statesman or stateswoman, like the former Tory Chancellor, Ken Clarke, or the former Labour deputy leader, Harriet Harman.

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Why stop there said cynics on social media? Perhaps ex-party leaders, like David Cameron, Ed Miliband and Neil Kinnock could be drafted to lead the coup against Brexit. Or the Queen herself? What about Gary Lineker and Greta Thunberg? The GNU hunt descended into farce and ridicule, as commentators and politicians, started playing caretaker fantasy football.

Following Caroline Lucas' call for a cabinet of her BFFs (Best Female Friends), we could perhaps have a cabinet of National Treasures, led by David Attenborough, with Patrick Stewart, Judi Dench and Gandalf. My favourite came from the New Statesman's political editor, Stephen Bush. He suggested a caretaker cabinet of Glasgow University alumni, led by Mhiari Black, on the grounds that Oxford graduates got us into this mess and should be excluded.

This is all great fun, but takes us no nearer to resolving this complex constitutional issue, which threatens to wreck the UK system of government. What happens when parliament, which is supposed to be sovereign, profoundly disagrees with the government on a matter of grave national importance? The legislature and the executive are on collision course over the right or otherwise of the Prime Minister to plunge Britain into a No Deal Brexit against the wishes of most MPs.

Governments govern and parliaments give consent - that's how the relationship is usually described. But the government of the day is a product of parliament. We elect political parties in our system, not presidents. A Prime Minister may exercise the residual powers of the monarch, making the laws of the land, but only if he or she can command the confidence of the House of Commons by winning a majority there.

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Currently Boris Johnson has a Commons majority of one, which is effectively less than zero, since around a dozen Tory MPs, led by the former Attorney General, Dominic Grieve, seem willing to vote against the PM in a Vote of NO Confidence (VONC) when parliament returns in September.

A VONC comes before GNU. Under the fixed term parliament act, if a government loses the vote, MPs have 14 days to put together an alternative government. If they fail, the defeated Prime Minister must call a general election. But the rules are vague on exactly when the general election should take place, and Mr Johnson is thought to be planning to delay polling day until after Britain falls out of the EU on 31st.

That of course would be outrageous, but it could happen if the opposition parties don't get their act together. Last week did not inspire confidence. Throughout the three-year Brexit crisis, the opposition party leaders have rarely been capable of see beyond narrow party advantage. Many MPs wanted a Norway-style compromise keeping Britain in the EU single market, but their parties couldn't co-operate long enough to make it a reality. Now, they are falling out over the GNU, which threatens to be a Government of National Disunity.

It is even possible that parliament could fail to move that vote of no confidence in Boris Johnson when parliament resumes on the 3rd September. The ragged band of rebels in Change UK, led by Chuka Umunna, have already said they will not support Jeremy Corbyn. It may seem astonishing that these Remain MPs, who left their parties because they wanted to halt a No Deal Brexit, would actually fail to support the one course of action that could do that. But this is politics.

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Then there are the Tory rebels, led by Dominic Grieve. They also hate the idea of seeing Jeremy Corbyn in Number Ten almost as much as they loathe seeing Boris Johnson there. Mr Grieve says he will not “facilitate Jeremy Corbyn to become Prime Minister”.

Many Tories, and some Labour MPs too, worry that if they let Mr Corbyn into Number Ten, even as caretaker, they may not get him out again. After asking Brussels for an extension to Article 50, what's to stop the new PM seeking to negotiate his own Brexit deal with Brussels?

It is theoretically possible that Jeremy Corbyn could win a no confidence vote but fail to establish his own legitimacy thereafter as caretaker PM. He would not have won an election, after all, and most MPs in the Commons mistrust him. Boris Johnson might refuse to recognise Corbyn as Prime Minister on the grounds that the Labour leader himself would not survive a vote of no confidence.

We could end up in the extraordinary position of having not one, but two governments claiming the authority to speak for Britain. The sitting government of Boris Johnson and the caretaker administration of Jeremy Corbyn. Mr Johnson has the civil service on side and the diplomatic corps. Corbyn has the Speaker and most of the opposition.

Assuming he succeeds in ousting Boris Johnson, Corbyn might also try to force an snap general election before Brexit Day on 31st October, while also requesting that Brussels delay Britain's departure. But he might not win that election. Leave voters would be up in arms. “Tell them again” would be the rallying cry of Brexit campaigners. Boris Johnson might even be returned with an enhanced majority if voters feel that the opposition parties botched their coup.

The last time a government fell after a VOC was in 1979 when the SNP helped bring down the Labour government of James Callaghan. But that was relatively simple. The rules were clear and Britain was not then in the process of leaving the European Union. The Fixed Term Parliament Act of 2010 does not specify the mechanism by which this unique and unprecedented transfer of power to a caretaker administration would take place. The Queen has made clear that she is not going to sort it out for them.

To say we are in uncharted waters is an understatement. We are entering the rapids of the greatest constitutional crisis in a century at least. The country and parliament are deeply divided. Tempers are frayed and there is widespread mistrust. There is anger on the streets. The idea of a government of national unity may simply be impossible because the country, like parliament, is incapable of uniting.