David Cameron said on Tuesday that he will press ahead with a vote next year on constituency boundary changes that will benefit his party at the next election, even though Nick Clegg has said he will instruct Liberal Democrat MPs to vote down the measures.

Cameron said: "I am going to say 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."

The prime minister would not be drawn on whether Lib Dem ministers would be deemed to be in breach of the ministerial code if they voted against what was at the outset a government-agreed measure. But if he were to make such a dramatic ruling, he would in effect be spelling the end of the coalition.

The Liberal Democrat leadership withdrew their support for boundary reforms in a tit-for-tat move following the collapse of their cherished plans to introduce an elected Lords this week.

The parliamentary vote on boundary changes is due in autumn 2013, after the boundary commissions have published their final proposals. But the issue is bound to become a political flashpoint. Cameron is already under some pressure from the Tory right to face down the Lib Dems, and threaten an early general election. The alternative would be to end the coalition and run as a minority administration, but press the Lib Dems to vote on individual measures, a system known as confidence and supply.

It is more likely that Cameron will seek to embarrass the Lib Dems politically over their sudden opposition to boundary reform, but have to accept that they are not going to vote through the boundary changes, and carry on with the coalition.

It would be almost impossible for Cameron to win the boundary review vote in either the Commons or the Lords without the support of the Lib Dems since Labour and the minority parties are opposed to the way the reform has been implemented.

The equalisation of the electorates of parliamentary constituencies favours the Conservative party collectively, but is opposed by many individual Tory MPs who face losing their seats.

But the appetite to attack the Lib Dems over boundary reform was highlighted by Lord Tebbit, the former Conservative cabinet member. He criticised "the monumental, unspeakable incompetence of my Conservative colleagues who have allowed Mr Clegg and the BBC to present boundary change as being proposed by the Tories solely to gain an advantage for themselves".

Some Tories strategists fear that without the boundary changes it will be difficult to secure an overall Conservative majority at the 2015 general election.

The Labour general secretary, Iain McNicol, has written to Labour branches in the wake of Clegg's announcements, saying plans to reorganise Labour constituency parties in England and Wales will not now go ahead.

Labour MP Paul Farrelly, who is likely to lose his seat under the reforms, said Clegg should never have gone along with the plans. "He was clearly willing to swap lost Lib Dem seats in the Commons for senators elected from plum posiiton on PR lists to his new House of Lords," he said.

But in a sign of the determination to keep the coalition together, Oliver Letwin, the Conservative policy supremo, and Danny Alexander, the Treasury chief secretary, have started discussions this week about a revamped version of the coalition agreement that could be published after the party conferences.

Figures close to Clegg, such as David Laws, have stressed this should be a revisiting of existing policy objectives, rather than setting out new objectives. Laws recently argued at an Institute for Government seminar that "the absolutely last thing we need to be doing is finding lots of new policy ambitions. Instead the focus should be on implementing things we are actually doing, focus on important things that matter and then any policy problems we did not anticipate. The idea that we are going to run out of things to do after a year or two is massively delusional. Adding another 100 to 200 policies is not only not necessary, but a massive distraction from dealing with a massive policy agenda".

Before their party conference, the Liberal Democrats hope announcements will be made on a major social housing programme. The business secretary Vince Cable recently told a seminar that "the public sector balance sheet has to be used to leverage in private capital, particularly in housing."

The chancellor, George Osborne has already indicated that he is ready to use about £20bn of state funds to underwrite private investment in infrastructure projects. The programme might go some of the way to satisfy party activists that Liberal Democrats are influencing Treasury thinking on how to stimulate growth.