David Cameron said he would fight for the UK national interest in any EU Treaty talks after being pressed on the issue by his own MPs at PM's questions.The prime minister said "the more eurozone countries ask for, the more we will ask for in return", including safeguards about the financial sector.
But Labour leader Ed Miliband said the UK had been "left on the sidelines".
Shortly after the exchanges, Boris Johnson called for a referendum if the UK was asked to approve a new treaty.
The Conservative Mayor of London said the UK should oppose any change which created a "very dominant economic government" across Europe.
"If Britain was asked to sign up to such a thing within the 27 (all the members of the EU), it would be right to veto it and if we felt unable to veto it, I certainly think that it should be put to a referendum," he told BBC Radio 4's World at One.
However, he said the government could not "reasonably" have a referendum if the new arrangements were confined to the 17 eurozone countries only.