In recent days the Conservatives have made it clear that if they are the single largest party after the election, David Cameron will seek to survive as prime minister - even if it is clear that he lacks the Commons support needed to govern. The aim is to delegitimise a Labour minority government at birth by declaring victory and framing Cameron as the only acceptable PM. (Though some Tories hope they could persuade enough Labour MPs to abstain in any vote.)

A Conservative cabinet minister told the Sunday Times: "It’s a race to be the largest party. We will say: 'We’re legitimate, we’re the largest party, we should carry on.' If necessary, dare the others to vote down a Conservative government. We’ll bring forward a vote of confidence on our Queen’s Speech so they do the deed in plain sight, rather than meekly saying, 'I suppose your numbers add up, goodbye'."

In response, Labour have denounced Cameron for planning to "cling to power" even if it becomes clear he will be defeated in the Commons. An aide told the New Statesman: "All the noise coming out of the mouths of David Cameron and Nick Clegg is about how they can cling on to power even if their coalition loses its majority. Clegg has shown his true colours – he personally wants to get back into bed with Cameron even at the price of betraying the Lib Dems’ fundamental principle of protecting our future in Europe. "

The aide added: "David Cameron is showing he is in an incredibly weak position. He won’t talk about the big questions in this election, how to create an economy which works for working families, how to sustain our NHS, how to get a better future for young people. Instead, he is trying to focus all attention in these final days on the process question of what happens after the election rather the decision people have to make in this election.

"Just like he did on the morning of 19 September – where Cameron had the opportunity to speak for the whole country after the Scottish referendum – he is instead showing he is driven by internal weakness and external electoral pressure to act only on behalf of the Tory party."

The political attractions of the Tories' gameplan are obvious but it would ride roughshod over constitutional convention. The relevant passage from the Cabinet Manual (Paragraph 2.12) states:

Paragraph 2.12 Parliaments with no overall majority in the House of Commons Where an election does not result in an overall majority for a single party, the incumbent government remains in office unless and until the Prime minister tenders his or her resignation and the Government’s resignation to the Sovereign. An incumbent government is entitled to wait until the new Parliament has met to see if it can command the confidence of the House of Commons, but is expected to resign if it becomes clear that it is unlikely to be able to command that confidence and there is a clear alternative.

The key line is that an incumbent government is "expected to resign if it becomes clear that it is unlikely to be able to command" the confidence of the House. Therefore, if Labour, the SNP (who have pledged to vote down the Conservatives) and other anti-Tory parties have a majority of seats, Cameron should resign rather than invite inevitable defeat in the Commons.

The Cabinet Manual also makes it clear that the person most likely to be able to command the confidence of the House (in this case, Ed Miliband) then becomes prime minister - even if it is not certain they would be able to do so. Paragraph 2.8 states: "Prime Ministers hold office unless and until they resign. If the Prime Minister resigns on behalf of the Government, the Sovereign will invite the person who appears most likely to be able to command the confidence of the House to serve as Prime Minister and to form a government."

By convention, then, Miliband would become PM without the need for any formal deal with the SNP - on whom Labour's majority would likely depend.