Prime Minister David Cameron faces increasing pressure after two top Tory politicians called for a referendum on Britain's role within Europe if eurozone countries form a tighter union.

Speaking to the Spectator magazine, eurosceptic Northern Ireland secretary Owen Paterson said Britain was at risk of being permanently outvoted by a united eurozone bloc within the EU.

“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 [a referendum], yes, because I think the pressure would build up.”

And Boris Johnson piled on the pressure when he told the BBC'sWorld at One programme that a new EU treaty would require a referendum.

"Well, it's absolutely clear to me that if there is a new treaty at 27, if there is a new EU treaty that creates a kind of fiscal union within the eurozone, then we would have absolutely no choice either to veto it or to put it to a referendum," he said.

David Cameron faced a barrage of questions in the Commons from his own MPs about Britain's relationship with the EU at prime minister's questions. He pledged to show "bulldog spirit" when meeting European leaders meet on Friday and to safeguard British interests.

"The more that countries in the eurozone ask for, the more we will ask for in return," he said.

But speaking in the Commons Labour leader Ed Miliband accused him of going from "handbagging" Europe to "handwringing".

The bitter exchange came after Ken Clarke broke ranks with his leader over Europe, saying Friday's crucial EU summit was 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.

Earlier this week work and pensions secretary Iain Duncan Smith said a referendum should take place in the event of any “major treaty change”.