Cameron threatens to use UK veto on proposed EU Treaty changes

Cameron has insisted that if eurozone countries want to use the "institutions of Europe" to rescue the single currency, they will have to back a number of "British safeguards" in return.

During emergency talks in Paris on Monday, German Chancellor Angela Merkel and French President Nicolas Sarkozy renewed calls for reform on the treaty.

The aim would be to allow far tougher rules and sanctions governing the eurozone in future to reassure markets about the euro's long-term stability.

Cameron said he was heading to talks in Brussels later this week "to defend and promote British interests".

"Now, the most important British interest right now is to sort out the problem in the eurozone that is having the chilling effect on our economy that I have spoken about," he said.

The credit ratings agency Standard & Poor's has put 15 euro nations and the European Financial Stability Facility (EFSF) on a list of those at risk of being downgraded.

"That obviously means eurozone countries doing more together and if they choose to use the European Treaty to do that, then obviously there will be British safeguards and British interests that I will want to insist on.

"And I won't sign a treaty that doesn't have those safeguards in it, around things like, of course, the importance of the single market and financial services.

"Now, if they choose to go ahead with a separate treaty, then clearly that is not a treaty that Britain would be signing or would be amending but, of course, if they want to use the European institutions, then we will be insisting on the safeguards and the protections that Britain needs.

"Britain is a member of the European Union, we benefit from our membership of the European Union. Key to that is the single market.

"We want the single currency crisis solved but we will protect and defend British interests at the same time."

Asked if he was prepared to reject a treaty that would save the single currency, Mr Cameron said: "To save the single currency you need more than a treaty, you need to address the competitiveness problem, the deficit problem, you need to take action to convince the markets that you are serious.

"You need all the institutions of the eurozone, including the European Central Bank, to get behind that.

"What I'm saying is that if - and eurozone countries do need to come together, do need to do more things together - if they choose to use the European Treaty to do that, Britain will be insisting on some safeguards too.

"As long as we get those, then that treaty can go ahead. If we can't get those, it won't."