The party with the most MPs gets to form the government.
In a hung Parliament, there's no rule that the largest party forms a government. It might get the first chance to try, but the incumbent can also claim this.
"The next government won't depend on who is the largest party... governments in the House of Commons are about who can command a majority."
Nicola Sturgeon, 24 April 2015
"The biggest party gets to form the government"
Jim Murphy MP, 7 March 2015
The leader of the Scottish National Party said on Newsnight last night that she will attempt to keep the Conservative Party out of government after the election, even if it has up to 40 seats more than any other party.
But Scottish Labour has said previously that the largest political party forms the government. Is this right?
In these terms, no. Labour are correct in saying that the party with the most MPs after every election since 1929 has proceeded to take power. But as Nicola Sturgeon says, there's no rule preventing a smaller party or parties taking over either.
Confidence of the Commons
The most important thing about forming a government is that it must "command the confidence of the elected House of Commons". When one party has an overall majority—that is, more MPs than every other party put together—this condition is obviously satisfied.
So in the event that Labour won an overall majority in May, the current government would have to resign and the Queen would invite Ed Miliband to become Prime Minister.
A House divided
But the more likely scenario—according to the opinion polls—is a hung parliament, in which no single party has an overall majority of MPs. This happened in 2010, resulting in the current coalition. What then?
If the negotiations after that result produce a viable new government between two smaller parties—for instance, a Labour/SNP coalition—that's the government we get. It doesn't have to include the largest party.
That's easy. But what's more complex is the dynamics of the game of thrones that would follow the election of a hung Parliament. As there could be a number of viable political combinations, and a government can stay in power with fewer than a majority of MPs actively supporting it, the party leader with the first opportunity to form the government has an advantage.
So are there rules about who gets the first attempt at trying to "command the confidence" of the Commons?
Incumbency trumps size
In the short term, the incumbent Prime Minister is entitled to remain in office to see whether he or she has enough support to stay on. In 1974 the Conservative Prime Minister Edward Heath tried to form a new coalition government even after an election that left the opposition, Labour, the biggest party.
David Cameron would be entitled to do the same, even if outnumbered by Labour (or by Labour and the SNP). He only has to resign before a vote to test the confidence of the House if it were clear that he no longer has it.
That said, in 2010 the Conservatives took the initiative in negotiating with the Liberal Democrats even though Labour, the incumbents, could technically have put together a viable combination.
More recently, the leader of the Democratic Unionist Party said on 29 March that, while he would be open to a deal with either Labour or the Conservatives, "in the first instance constitutionally we would want to go to the party that has won the most seats".
As academics from the UCL Constitution Unit point out, it's no longer clear whether size or incumbency is the deciding factor in who gets first dibs on trying to form a government. Ultimately, the attitude of the various political parties will establish this.
A political rather than legal process
In the context of the current election, it could be that the Conservatives don't get an overall majority but end up as the incumbents and the largest party—so Labour might accept their right of first opportunity to form the government. Remember, though, that if they fail it's perfectly legitimate for two smaller parties to form a coalition.
The most obvious potential for dispute is where the Conservatives end up with slightly fewer seats than Labour, who could say that they should get first opportunity based on size, while the Conservatives say that they get first opportunity based on incumbency.
Experts are divided on what, as a matter of strict constitutional law, should happen then. One school of thought says that the Queen should have a role to play in resolving the deadlock, while others feel that it's entirely a matter for the politicians to argue out among themselves.
But in some respects there's no such thing as strict constitutional law in the UK, and hard political facts ultimately override many of these subtleties. A Labour/SNP bloc that could out-vote the Conservatives in the House of Commons would, if it wanted to, be able to eject them eventually in a vote of confidence.
As the academic J.A.G. Griffith famously put it, "the constitution is no more and no less than what happens".
Update 17 March 2015
Scottish Labour have been in touch about this issue. The party says that "our point refers to election outcomes, rather than procedure or process", pointing out that in practice, the largest party has formed the government after every election since 1929. Therefore, Scottish Labour say, it's accurate to state that "the largest party forms the government after the election. That has been the case in every UK election since women got the vote".
A claim that the largest party—with or without an overall majority—generally forms the government, rather than must form the government, is perfectly accurate. A claim that no government can legitimately be formed without including the largest party is not.