Image caption Mr Cameron has said the government must focus on the economy

David Cameron has said he will press ahead with proposed changes to the House of Commons despite the Lib Dems saying they will vote against them.

The prime minister said plans to redraw constituency boundaries would be "put forward" to MPs and urged all parties to back the "very sensible" proposals.

Senior Lib Dems have said they will oppose them after Tory MPs blocked proposals to reform the House of Lords.

They say this meant the Tories "reneged" on the coalition agreement.

Changes to the Lords - the plan was to make 80% of peers elected and to halve the number of members to 450 - were a long-held goal of the Lib Dems.

But more than 90 Conservatives defied the government in a vote on the issue in July and Mr Clegg pulled the plug on the plans on Monday.

'Frustrating'

Speaking on a visit to a children's activity centre in Wales, Mr Cameron said it was "frustrating" that the Lords plans had been dropped in the face of opposition "from Labour and others" in Parliament.

All of these claims about the coalition being on its last legs are simply not true Jeremy Browne, Foreign Office minister PM to stick to Welsh reforms

But he said he could not allow "month after month of wrangling" over the plans and there would now be extra "space" to concentrate on the government's priority of the economy

The climbdown over Lords reform has thrown into doubt its plans to reduce the size of the Commons from 650 to 600 and redraw constituency boundaries to make them roughly the same size.

Mr Cameron, whose party are seen as most likely to benefit from the changes to the Commons, said that "obviously we want the boundary vote to go ahead".

"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'," he said.

"I think everyone should come forward and vote for that proposal because it is a very sensible proposal and it will be put forward."

When he appeared before a committee of MPs earlier this year, Mr Clegg said there was no link between Lords reform and the boundary changes.

But Lib Dem Foreign Office minister Jeremy Browne said on Tuesday that they were "part of the same constitutional package".

He told Radio 4's Today programme he would oppose boundary changes in a vote expected to take place next year but insisted the dispute would not wreck the coalition.

"Let's get on with the huge areas where we do agree and where we think we can work constructively in the national interest," he said.

"All of these claims about the coalition being on its last legs are simply not true. We can get on with that huge body of work instead."

'Electoral disadvantage'

The BBC's political editor Nick Robinson said the Lib Dems were facing up to the prospect of not being able to deliver any constitutional change of any significance while in government.

For the time being, at least, life in the Lords will carry on as normal Sean Curran, BBC Parliamentary Correspondent Read Sean's full analysis on the future of the Lords

The Conservatives, meanwhile, may have to live with the current parliamentary boundaries which meant it took many more thousands of voters to elect a Conservative MP than a Labour MP.

Tim Montgomerie, editor of the Conservativehome website for party activists, suggested losing the boundary changes would be a "huge blow" to the Conservatives' chances in the next election.

He told the BBC News Channel: "If you look at the electoral system, Labour can get a parliamentary majority with just a 3% lead in the opinion polls.

"The Conservatives need an 11% lead to get the same result. And one of the reasons for that is that Labour seats tend to be smaller in size than Conservative seats.

"Boundary reforms would not have reduced all of the disadvantage in the electoral system but would have given the Conservatives about 20 more MPs."

Labour has said the boundary changes were "arbitrary" and designed to benefit the Conservatives rather than improve the political system or save money. The party says Mr Clegg did not oppose them when they were agreed in principle by Parliament last year.