Nick Clegg has warned David Cameron that a promise to hold a referendum on membership of the European Union is against the national interest and risks derailing the fragile UK economy.

The deputy prime minister criticised the prime minister as opinion polls in two Sunday newspapers suggested the Tories had received an opinion poll bounce from the policy.

In a pledge seen as acknowledgement that his own backbenchers are under pressure from the UK Independence party, Cameron said on Wednesday that he wanted to stay in the EU but pledged to claw back powers and offer voters a choice of a new relationship or exit by the end of 2017 if the Conservatives are returned to power in 2015. His stance has delighted Eurosceptic backbenchers.

Clegg dismissed the prospects of securing a significant renegotiation and suggested Cameron should be concentrating on the economy – which risks slumping into a triple-dip recession.

"My priority will always remain a simple objective of building a stronger economy in a fairer society, enabling everybody to get on in life," the Liberal Democrat leader told BBC1's Andrew Marr Show.

"And I think that job is made more difficult if you have years and years of tying yourself up in knots having arcane debates about the precise terms of the membership of the European Union before we get to a referendum.

"It is not in the national interest when we have this fragile recovery, when we have a very open economy which is very dependent on investors in the car industry and the banking system and so on.

"You must always, when you are trying to piece together a recovery, foster those precarious conditions of greater confidence in the economy. You mustn't do anything to make that more difficult."

He claimed that most businesses he had spoken to were concerned by the uncertainty of a possible future referendum and its consequences.

"Is it deliverable to in effect say to the rest of the European Union: we want to do all the bits that we like but can you keep all the stuff that we think is not good for the economy? That is not plausible," he said.

He denied his party had reversed its position by opposing a referendum – having argued strongly for an in-or-out vote in 2008 – saying that had been in the context of the approval of the Lisbon treaty.

"It is the Conservatives who have decided to completely reinvent the wheel and tie the country up in knots by redesigning and fiddling around with the terms … before the British people even get near a referendum," he said.

Grant Shapps, the chair of the Conservatives, said the government was not ignoring the country's perilous economic position by developing a new policy on Europe. "Europe is a big central issue which has a major affect on most people's daily lives and should not be ignored," he told Dermot Murnaghan on Sky News.

The Labour leader, Ed Miliband, defended his decision to at present oppose the referendum promised by the prime minister.

"I just don't think it's in the national interest. Right now it doesn't make sense to commit to a referendum years ahead.

"Has he [Cameron] changed his mind because he's changed his view about the national economic interest? Or has he changed his mind because there's pressure in the Conservative party and he's worried about Ukip?

"I contend it's the second, not the first. That's why I've stuck to the positioning we've always had," Miliband said.

A ComRes poll for the Sunday Mirror and the Independent on Sunday has put the Tories up five points on 33%. Labour remains unchanged on 39% but its lead has slipped from 11 points to six over a month.