If the prospect of government MPs tabling an amendment to the Queen's Speech wasn't unusual enough (it hasn't happened since 1946), it now appears that David Cameron may be prepared to take the extraordinary step of supporting them. The Sun reports that Cameron is ready to vote in favour of the Conservative amendment, which "Respectfully regrets that an EU referendum bill was not included in the gracious speech". A No. 10 source tells the paper: "The PM is determined to make as many people as possible aware how keen he is to hold this referendum.

"This amendment backs up his policy, which is a Conservative Party policy, so why shouldn’t he vote for it too?"

In other words, the Prime Minister may be about to rebel against his own government. That really would put us in uncharted territory. As the Parliament website states, by convention, "If the Queen’s Speech is amended, the Prime Minister must resign." The last time an amendment was successful was in 1924 when Labour tabled a motion of no confidence in Stanley Baldwin's Conservative government. After the motion was passed by 328 votes to 251, Baldwin resigned as prime minister and Ramsay MacDonald formed the first Labour government.

With Labour and the Liberal Democrats set to vote against the amendment (they have 314 MPs to the Tories' 305), there's almost no chance of it passing (although at least two Labour MPs, John Cryer and Kelvin Hopkins, have signed the amendment and there's always the option of abstaining...). But were the Tory rebels successful, it is clear that Cameron would either have to resign or abandon centuries of parliamentary convention.

Update: It look as if there may be an escape route for Cameron. I've just spoken to the Commons Information Office which has informed me that as a result of the Fixed-term Parliaments Act, a successful amendment to the Queen's Speech is no longer regarded as a vote of no confidence in the government. This is because, for the first time, the bill offered a legal definition of a no confidence vote - a motion stating that "That this House has no confidence in Her Majesty’s Government." - meaning that defeats on matters such as the Queen's Speech or the Budget are no longer regarded as votes of no confidence in the government. Prior to the act, as the Information Office put it, "it was a motion of no confidence if everyone agreed that it was a motion of no confidence."

A 2010 briefing note from the House of Commons Library had suggested that some ambiguity remained. It stated that it was "not clear whether a defeat on a motion or issue of confidence would count as a vote of no confidence for the purposes of the legislation. For example, it is not clear whether a defeat on the Government’s budget would be considered as a vote of no confidence." It went on to suggest that "One possibility would be for the Government to make it clear before such a division that they considered it to be a matter of confidence; then the Speaker would certify it as such. This would effectively allow the Government to table a constructive vote of no confidence."

But the Commons Information Office confirmed to me that this was not an option legally available to the government.

I asked earlier whether, rather than resigning, Cameron would abandon centuries of parliamentary convention. It turns out he already has.