Tory MP David Ruffley broke cover at the weekend to warn David Cameron  that his leadership would be at risk if the Conservatives performed poorly in next year's European elections. He told Sky News's Murnaghan programme: "I think next May's Euro elections might put pressure on him to go harder because there is a lot of speculation in and around Downing Street, so I am led to believe, that Ukip might come first.
"Now if that happens next May there'll be 12 months before the election and some of our colleagues in marginal seats might get a bit windy. I don't think UKIP are going to win seats but they could split the Conservative vote if they are strong and let Labour through in those marginal seats."
Over at the Telegraph, Benedict Brogan suggests that  the threat of a putsch is real, reporting that the Conservative whips believe "there is a hard core of about 30 irreconcilables who will do anything to bring down Dave".
So how would Ruffley and his colleagues go about the putative regicide? Under current Conservative rules , a vote of no confidence is triggered when at least 15 per cent of Tory MPs ("in receipt of the Conservative whip") write to the chairman of the backbench 1922 Committee (currently Graham Brady ) requesting one. This can be done either collectively or separately and the names of the signatories are not disclosed. With 305 sitting Conservative MPs, 46 signatures would be required for a vote to be held. Once this threshold has been met, the chairman in consultation with the leader then determines the date of such a vote "as soon as possible in the circumstances prevailing".
If the leader wins the support of a simply majority in the vote, they remain leader and no further vote can be held for 12 months from the date of the ballot. If they lose the vote (again, on a simple majority basis), they must resign and may not stand in the leadership election that then follows. Unlike in 1989, when Tory backbencher Anthony Meyer stood against Margaret Thatcher, no "stalking horse" candidate is required to oust the leader. While Cameron would easily win any vote, he would be damaged if a significant minority of MPs either voted against him or abstained. In 1989, Thatcher defeated Meyer by 314 votes to 33, but once spoilt ballots and abstentions were included, it emerged that 60 MPs - 16 per cent of the parliamentary party - had failed to support her. In Meyer's words, people then "started to think the unthinkable".
Under the current Conservative leadership election rules, adopted in 1998, if there is only one valid nomination, that person is elected. If there are two, both candidates go forward to a vote of the party membership. If there are three or more, a ballot is held within the parliamentary party to determine the two who go forward to the membership.
In 2005, in the final act of his leadership, Michael Howard attempted to change the rules in order to give MPs, rather than party members, the final say. The move was prompted by the 2001 leadership election, which saw the popular Ken Clarke win the MPs' vote but Iain Duncan Smith trump him in the members' ballot. Unsurprisingly, after Duncan Smith's calamitous time as leader, most felt a Clarke victory would have served the party better. But Howard's proposals failed to win  the two-thirds majority required, with only 58 per cent of activists endorsing them (although 71 per cent of MPs did), and the status quo prevailed.