David Cameron has been plunged into a row with his Liberal Democrat coalition partners after he raised the prospect of Britain exploiting the eurozone crisis to begin reclaiming powers from Brussels.
On the eve of a crunch Commons vote on holding a referendum on Britain's future in the EU, the Prime Minister promised to "exact a price" if the eurozone countries sought closer integration to deal with the problems of the single currency.
However, his comments drew a stinging rebuke from Deputy Prime Minister Nick Clegg who insisted that it was "far too early to speculate" on what the Government position would be in the event of a proposed treaty change.
Speaking at the end of an EU summit in Brussels, Mr Cameron said there was a "possibility" that treaty change could be on the agenda as early as the December gathering of EU leaders.
"Treaty change can only happen if it is agreed by all the 27 member states of the European Union," he said. "Any treaty change - as the last treaty change did - is an opportunity for Britain to advance our national interest. The last limited treaty change, which brought about the European stability mechanism, gave us the opportunity to get out of the euro bailout fund that the last government opted into.
"We used that to advance our national interest. Any future treaty change we would use to advance our national interest once again."
However, a spokesman for Mr Clegg said: "It is far too early to speculate on the Government's negotiating position in a future inter-governmental conference which may well not happen and to which many countries would object.
"Nick Clegg believes that the UK's national interest is best served by developing a reformed, competitive, open and productive European economy. That should be the focus of any future EU discussions."
The vote threatens to reopen deep wounds within the Conservative Party over Britain's position in the EU, raising the prospect of a return to the Tory civil war of the 1990s. So far, 60 Tory MPs have now signed the main motion while another 33 have signed compromise amendments which ministers say also run counter to Government policy.
There have been reports of the Conservative whips threatening potential rebels that they will damage their chances of a ministerial job or lose their seats when parliamentary boundaries are redrawn in a bid to limit the scale of the revolt.