What makes a government legitimate?

As polling day nears with no sign of a breakthrough in the polls, the question on many lips is not only what kind of government will we get, but will that government have legitimacy?

David Cameron has suggested that a Labour government relying on Scottish National Party support would be a "con trick" in the light of Ed Miliband's disavowal of a deal with the nationalists. Nick Clegg has said that a minority administration excluding the Liberal Democrats would be "messy" and unstable and lead to a second election before Christmas.

But SNP leader Nicola Sturgeon has also raised questions about any administration which does not feature her party - expected to take the lion's share of seats north of the border - arguing that one test of the government's legitimacy should be whether "it reflects the whole of the UK".

Cameron plotting to claim he's won?

Speculation continues to grow that David Cameron may try to proclaim he's won even without a majority. Labour sources are predicting that the prime minister will attempt to seize the advantage and stay in Downing Street. In these circumstances it's unclear what Labour will do other than vote down a Queen's Speech and block a Tory budget in an attempt to form an alternative government.

In the absence of either a written constitution or a lengthy history of coalition government, Britain's political classes are to an extent groping in the dark as they grapple with the practical implications of the result they are likely to be handed on Friday. Forming a new government - or plunging the country into a second poll - could take weeks.

And even after they have thrashed out a solution that works in the cloisters and corridors of the Palace of Westminster, the danger remains that it will not be accepted by the public as a fair reflection of the votes they have just cast.

The Cabinet Manual drawn up in 2011 by former Cabinet Secretary Lord O'Donnell, who oversaw coalition negotiations after the last election, states that: "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."

Lord O'Donnell today confirmed that this means it would be "feasible" for Mr Cameron to stay on until the State Opening of Parliament on May 27, even if he is "well short" of the 326 Conservative MPs needed to secure an overall majority in the Commons. This was a "political judgment" for the PM, who could face questions if he sought to hang on in the face of probable defeat.

First refusal

University of Birmingham constitutional history expert Dr Matt Cole said there would be considerable pressure on Mr Cameron from within his party to "claim the premiership" if Conservatives emerge as the largest group in the new Parliament, even if they cannot establish an overall majority with the help of the Liberal Democrats.

If Mr Cameron chose to press ahead with a Queen's Speech, only to fall victim to a vote of no confidence, an alternative leader would have 14 days to show he or she can command the confidence of a majority of MPs. While that alternative leader is almost certain to be the head of the main opposition party, Dr Cole said there remained room for dispute over who should get "first refusal".

And he said: "The manual doesn't resolve the question of what we mean by 'commanding confidence'. Does the tacit abstention of another party count as command? Does a 'confidence and supply' arrangement count as command?

"If there is a large difference in numbers of MPs between the party which can win a vote of confidence and another larger party which - because it does not have enough friends in the Commons - cannot, does that undermine the legitimacy of the arrangement?

"Even if we can identify who, according to constitutional procedures, ought to be prime minister, that might not enjoy public acceptance."

Recent polling suggests Mr Cameron may well have the largest team of MPs at Westminster, but that Mr Miliband may be better placed to survive a confidence motion on the back of votes from SNP MPs with whom he refuses to enter coalition.

"We are faced with the prospect that the incumbent Prime Minister has as large a share of the vote as last time and quite close to the same number of MPs, and the leader of the opposition has quite clearly failed to match that, and yet the leader of the opposition becomes prime minister, even without incorporating another party in a coalition," said Dr Cole.

Uncharted territory

If a prime minister does manage to get through a confidence vote, he will not have the luxury enjoyed by Harold Wilson in 1974 of calling a second election at a time of his choosing in the hope of securing the kind of mandate that would firmly establish his legitimacy.

New rules state that this can be done only with a two-thirds majority of MPs, and with none of the parties likely to be financially ready for another campaign, this would be like asking "two-thirds of turkeys to vote for an early Christmas".

So the public may be stuck for five years with a government which they don't feel they have chosen and which does not include the party which arguably has the largest mandate to govern. History provides very little guide as to how they would react.

And doubts over the government's legitimacy may be heightened by the very compromises which have to be made for it to be created, said Dr Cole.

"If you ask me for a prediction, it is that in the formation of a new government, politicians will do something they have promised not to, and the new government will carry the burden that it is based on a broken promise."