Tory patience begins to run out after Clegg's 'act of betrayal'

David Cameron won a noisy show of support from his newly enlarged group of Tory MPs for his final offer to the Liberal Democrats.

In a packed private meeting addressed by the Conservative leader, they cheered, laughed and banged their desks as he set out his party's negotiating red lines. But beneath the surface unity, there was dismay, anxiety and barely disguised fury at what many saw as straightforward betrayal by the Liberal Democrats. One said simply: "They are a bunch of tarts selling themselves around."

Away from the official briefing of journalists, two backbenchers confronted Oliver Letwin, a member of the party's negotiating team, over the Tory offer to the Liberal Democrats of a referendum on changing the voting system. One former minister said he doubted a deal could now be done with Nick Clegg's party, adding: "The trouble is that Labour are offering them the whole cake. Why would they take half a cake from us?"

The problem for Mr Cameron is that the overwhelming majority of his party are strong supporters of first-past-the-post for Westminster elections. Another ex-minister described any move towards electoral reform as "a deal-breaker for most of us". He added: "We have been promised that if we ever got to a referendum, all of us would campaign against change. Almost every Conservative would."

When talks between the Liberal Democrat and Conservative negotiators ended just after 11am, Mr Cameron's team believed they had made solid progress towards a deal. They acknowledged that agreement was far from in the bag, but said the "mood music" between the two sides was promising. The impression was reinforced during a face-to-face meeting yesterday morning between Mr Cameron and Mr Clegg.

The Tory leader called a meeting of his Shadow Cabinet shortly afterwards. He refused to go into detail – to the frustration of some of those present – but reassured them that the talks were going smoothly.

Hours later, as the meeting of Liberal Democrat MPs broke up, the Tory team realised they faced new problems. And when they learned of Gordon Brown's dramatic move – during a second Shadow Cabinet meeting – they knew their negotiations with Mr Clegg's party were in crisis. At that point they decided to unveil the referendum offer, a move that Tory sources insist is their final offer to the Liberal Democrats.

Mr Cameron went straight to address the party's 1922 backbench committee, the first parliamentary meeting for at least one-third of those present. He received cheers and applause as Patrick McLoughlin, the Tory chief whip, introduced Mr Cameron as the Tory leader who had delivered the most significant increase in the number of MPs for 80 years.

Mr Cameron, speaking without notes, opened the hour-long session, in a committee room overlooking the Thames, by setting out the detail of the party's offer to the meeting. The leadership was reassured by the fact that MPs ranging from Stephen Dorrell on the left of the party to John Hayes on the right stressed their support for his negotiating stance. Tory sources said "one or two" asked questions, but none opposed the strategy.

Afterwards George Osborne, the shadow Chancellor and one of the Tory negotiating team, denounced Gordon Brown's suggestion of a "rainbow coalition" and said it was "not in the national interest". He added: "In good faith we are making an offer to the Liberal Democrats of a strong and stable government with a considerable parliamentary majority, in coalition, and with a referendum on the alternative voting system. The Liberal Democrats want to change our voting system and we are making that offer.

"So the parliamentary party, in the extraordinary meeting where we genuinely consulted them following a meeting of the Shadow Cabinet, has pretty much to a person – I can't think of a single person who objected – endorsed the position set out and agrees to make this offer now to the Liberal Democrats as a final offer."

The message was reinforced in a statement by the shadow Foreign Secretary William Hague, delivered outside the St Stephen's entrance to the House of Commons. "It is urgent that the country has a new and stable government," he said. "The choice before the Liberal Democrats... is whether to go in with the Labour Party in a government that would not be stable or secure, because it would rely on other minor parties for any parliamentary majority at all; that would have a second unelected prime minister in a row – something we believe would be unacceptable to the great majority of people in this country – and which would impose voting reform without any consultation with the people of the country, something we believe to be profoundly undemocratic.

"Or they can choose to continue their talks with us, to make a coalition with the Conservative Party, which is on offer, in a government that would have a stable and secure parliamentary majority; a majority of 76 in the House of Commons, something highly desirable in our current economic situation; that would have an elected prime minister in David Cameron, the leader who obtained by far the most votes and seats in the general election held last week; and which would say that any reform of our voting system must be subject to a referendum of the people of this country."

Asked to describe the mood of the meeting, Michael Gove, the shadow Schools Secretary, said: "Loyal. Immensely understanding. Appreciative of what David did during the campaign. They recognise that David has changed the party in order to help them get to this point and there is trust and respect for the way he has handled this." Told about Mr Osborne's claims of unanimous support for the negotiating strategy, one of the backbenchers who confronted Mr Letwin said: "I wasn't given a chance to speak, so I don't know how he can say I'm happy with it."

Other MPs said they could live the offer being made to the Liberal Democrats. Richard Ottaway, who is seeking to become chairman of the 1922 committee, told the London Evening Standard: "In order to form a government, there may have to be some movement on something. If it is an alternative vote so be it. I have got an open mind on alternative vote." Peter Bottomley, the former minister, said: "The alternative vote which was Gordon Brown's last throw is not going to do serious damage to the country. I think people could live with that."