Nick Clegg has delivered his strongest rebuke so far to Tory eurosceptics hoping to reclaim powers from Brussels.
The Deputy Prime Minister warned that it would be “economic suicide” for Britain to “retreat to the margins” of Europe.
Rewriting the EU’s founding texts would open a “Pandora’s box” and leave the continent paralysed when it should be focused on restoring economic growth.
The Liberal Democrat leader’s intervention came with David Cameron under massive pressure to bring powers back to Westminster as part of negotiations for any new treaty.
More than 80 Conservative MPs rebelled last week over a call for a referendum on UK membership of the union. And polls have suggested that two-thirds of the public – and 80 per cent of Tory voters – agreed with the rebels.
Writing in a Sunday newspaper, Mr Clegg said it was clear the 17 eurozone nations were moving towards more fiscal integration and the European landscape was “about to change”. But he complained that “two extremes” were dominating the debate on how Britain should respond.
“On the one hand, there are some who see an opportunity for a more centralised EU, built around a tighter, quasi-federalist core,” Mr Clegg wrote. “On the other, are those who imagine a chance for the UK to draw away from the union.
“They relish the prospect of a unilateral raid on Brussels’ powers. The irony is that both options require treaty change: Europhiles and Europhobes are clamouring for the same thing.
“As always, neither extreme of the argument is right. Both would have the UK give up our place at the European top table, sacrificing the influence essential to our prosperity.
“It is only by having a loud voice in a united Europe that we can promote the open economy that will deliver growth.
“Being shoved to the margins, or retreating there voluntarily, would be economic suicide: a sure-fire way to hurt British businesses and lose jobs.
“Having worked in the EU for years, my concern is that tampering with the EU’s founding texts is opening a Pandora’s box, leaving us paralysed by ideological battles, institutional navel- gazing and special demands from every member state.
“These are dangerous distractions when our urgent priorities are restoring stability and jump-starting growth.”
Mr Cameron said the UK needed to prevent closer integration of the eurozone posing a threat to the single market.
“As the eurozone countries go ahead, as they must, trying to co-ordinate and combine more, I think it’s right that those countries not in the euro ask for some arrangements and guarantees that the single market is going to be properly protected.
“There is a concern that because the 17 are going off and doing more together, that could impact badly on the single market. We need to stop that from happening.”
Former Lib Dem leader Sir Menzies Campbell said there could be no unilateral renegotiation of powers by the UK.
“If Britain goes along saying ‘we want these powers back’, what about the other 26 members – will some of them want powers back?
“If they do, you then get into an exchange or a discussion which would go right to the very heart of the principle advantage of the European Union which is the single market.”