David Cameron has pledged to show "bulldog spirit" when European leaders meet on Friday.
The prime minister faced tough questions from his own party and from the opposition during Wednesday's PMQs, but pledged to safeguard British interests. "The more that countries in the eurozone ask for, the more we will ask for in return", Cameron pledged.
Labour leader Ed Miliband ramped up the pressure on Cameron ahead of the summit, accusing him of going from "handbagging" and promising to repatriate powers to "handwringing" over Europe.
The clash came as it emerged Eurosceptic Northern Ireland secretary Owen Patterson had told the Spectator an EU referendum is “inevitable” if there is closer fiscal integration in the eurozone.
“If there was a major fundamental change in our relationship, emerging from the creation of a new bloc which would be effectively a new country from which we were excluded, then I think inevitably there would be huge pressure for a referendum,” he said.
“I think there will have to be one, yes, because I think the pressure would build up.”
Justice secretary Ken Clarke broke ranks with David Cameron on Europe on Wednesday morning, saying that Friday's crucial EU summit is not a good time to be trying to extract concessions from Brussels.
A Number 10 spokesman said on Wednesday that any treaty changes will need to go through Parliament.
On Tuesday, Cameron warned he would block plans for a new EU Treaty unless European leaders agree to a list of British demands.
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