The Conservatives also refused to rule out sacking Liberal Democrat ministers who vote against a key Coalition policy in a sign of escalating hostilities yesterday.
It came as the Prime Minister called on parties of all shades to support "sensible" proposals that would cut the overall number of MPs from 650 to 600, and Scottish MPs from 59 to 52.
The Tory leader will have no parliamentary majority for the Government policy after the LibDems vowed to block them.
Deputy Prime Minister and LibDem leader Mr Clegg said on Monday he would order his MPs to vote against the changes, accusing Mr Cameron of a "breach of contract" over Lords reform.
The Tories could still push through the policy if they secured the support of a "rainbow Coalition" of most of the smaller parties, such as the SNP, Plaid Cymru and the Democratic Unionist Party (DUP).
The LibDems played down that possibility yesterday, insisting the plans were still "effectively dead".
Party sources said the numbers were so tight it would take a miracle for the Tory leader to win the vote.
They added that, even if Mr Cameron succeeded, he would later lose a vote in the Lords, where the LibDems together with Labour have greater strength.
It is understood the Prime Minister would have to offer significant concessions to smaller parties, many of which oppose the changes.
Plaid Cymru is thought to oppose cutting the number of MPs without a transfer of powers to the Welsh Assembly. The DUP has also condemned parts of proposals for new boundaries as "gerrymandering".
However, SNP sources said the party could be prepared to vote with the Conservatives.
Chief whip Stewart Hosie said: "After arguing for boundary changes on principle it is shameless of the LibDems to ditch these reforms to settle scores with the Tories.
"The boundary review had nothing to do with Lords reform and these shenanigans show why Westminster cannot be trusted on the key constitutional issues.
"Given these reforms were being driven by Nick Clegg, we need answers from David Cameron on how and when he intends to put this to a vote."
The Prime Minister earlier outlined his plea for cross-party support for the plans, which would also change the shape of almost every constituency in the UK to try to equalise voter numbers.
Mr Cameron said: "I am saying to every MP 'look, the House of Commons ought to be smaller, less expensive and we ought to have seats which are exactly the same size'.
"I think everyone should come forward and vote for that proposal because it is a very sensible proposal and it will be put forward."
LibDem aides insisted the party would not change its position.
Last night Downing Street sources refused to rule out the possibility that ministers could be sacked for voting against Government policy – despite the fact such a nuclear option could effectively lead to the end of the Coalition.
A source said that "we will cross that bridge when we come to it".
Jeremy Browne, a Foreign Office minister, called for unity. He said: "It seems to me to make a lot more sense, if there's an area we cannot agree on, that we put that to one side, we accept we can't agree on that and we get on with working together on all the areas we do agree on."
LibDems and Labour, who oppose the boundary plans, have 311 votes combined, with the Conservatives on 305.
Excluding Sinn Fein, who refuse to take their seats, along with the Speaker and deputy speakers, 321 votes are needed for a Commons majority. Of the smaller parties, the DUP have eight MPs, SNP six, Plaid three, the SDLP three, independents two, and the Alliance, Greens and Respect one.
A November by-election in Corby following the resignation of Tory MP Louise Mensch could also change the parliamentary arithmetic.
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