Prime Minister David Cameron is to travel to Brussels for a major EU summit on the eurozone debt crisis.
Germany and France want a new EU treaty with measures to stop a repeat of the problems threatening the euro's future.
Mr Cameron is under pressure from many Tory MPs to resist moves to strengthen the power of Brussels over EU members.
Tory ex-foreign secretary Sir Malcolm Rifkind has backed a tough approach - but urged Mr Cameron to stick to issues relevant to the crisis.
In a joint letter on Wednesday, France's President Nicolas Sarkozy and Germany Chancellor Angela Merkel called for the 17 eurozone countries to have common corporation and financial transaction taxes.
Meanwhile at Prime Minister's Question Time, Mr Cameron said he would fight for the UK national interest in any EU Treaty talks, following questions from Conservative MPs.
He said: "The more eurozone countries ask for, the more we will ask for in return," including safeguards about the financial sector.
Earlier, Downing Street said any treaty signed by the UK "will need to go through Parliament". It has said that a referendum will not be necessary because the proposed changes would not involve a big shift in power from London to Brussels.
But Labour leader Ed Miliband said at Question Time that the UK had been "left on the sidelines".
London Mayor Mr Johnson 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.
Sir Malcolm said Mr Cameron needed to focus on the matter in hand: "The tough approach required from David Cameron is perfectly reasonable if he concentrates on issues relevant to this crisis, such as the potential threat to the City of London.
"But if we were to have debates and discussions on Friday about whether we should repatriate the Common Fisheries Policy or the Working Time Directive or matters of that kind, the rest of the world would be aghast."
More than 80 Tory MPs defied the government last month and called for a referendum on the UK's membership.