Prime Minister David Cameron insisted Britain is not "turning our backs on Europe" as he met fellow EU leaders for the first time since announcing his plan to stage an in/out referendum on UK membership.
Mr Cameron held brief talks with several European leaders at the World Economic Forum in Davos, Switzerland, where he issued a warning that the EU must change because it is being "out-competed, out-invested and out-innovated" by rivals around the globe.
But in London, Deputy Prime Minister Nick Clegg renewed his assault on Mr Cameron's "vague" and "implausible" plans to renegotiate the terms of Britain's EU membership before staging a referendum by the end of 2017.
Mr Clegg told LBC 97.3 radio: "I simply don't understand the point of spending years and years and years tying yourself up in knots, so-called renegotiating the terms of British membership in ways which at the moment at least are completely vague. I think that discourages investment and inhibits growth and jobs, which have to remain our absolute priority at a time when the economy is still struggling to recover."
The aim of any renegotiation was unclear, he said. "Either it's just basically a bit symbolic, so you tweak the working time directive and a social law here and an environmental law there, which everyone will agree with - in which case what's all the fuss about? Or you're going to do something which I think is wholly implausible, which is basically to totally rewrite the rules to benefit us and disadvantage everybody else, which is clearly not going to be agreed to."
In an interview with CNN, Mr Cameron insisted that the Government was not putting a list of demands on the table and threatening to storm off if they were not met.
He said: "What we've said is we think there's a whole range of areas where the European Union has legislated too often and gone too far, covering areas like social and employment legislation, environmental legislation.
"A whole series of areas. I mean, just one example, the hours that hospital doctors work in Britain is, you know, dictated sometimes by rules far in Brussels. That really isn't necessary in an open, flexible, competitive Europe.
"We're not putting a list of demands on the table and saying we'll storm off if we don't get them.
"What we're saying is we should in Europe have changes that will benefit all of the countries of the European Union, but which at the same time will, I think, make Britain more comfortable with her place in the European Union."