David Cameron today pledged to do "everything possible" to stop signatories of a new treaty from using the EU's institutions to do their business behind the UK's back.
The Prime Minister admitted there were "legal difficulties" but insisted any new treaty should be about fiscal union and not the single market.
Institutions such as the European Commission and European Court of Justice should not be used to carry out the work of those who sign a new treaty, he added.
Mr Cameron also denied that he was unprepared for last month's summit at which he refused to sign the UK up to any new treaty.
He told BBC Radio 4's Today programme that he had met German Chancellor Angela Merkel three weeks before the summit to set out the UK's position, which had been agreed "across Government".
Mr Cameron said: "Part of the problem is that the legal position is unclear.
"One of the strengths of there not being a treaty within the European Union is that the new thing, whatever it is, can't do things that are the property of the European Union.
"They shouldn't be doing things that are about the single market or about competitiveness, and we will be very clear that when it comes to that you cannot use the European institutions for those things because that would be wrong.
"You can't have a treaty outside the European Union that starts doing what should be done within the European Union and that goes back to the issue of safeguards."
He added: "There are legal difficulties over this. One of the problems is that the European Court of Justice, we all think it is great independent arbiter, but the European Court of Justice tends to come down on the side of whatever more Europe involves.
"Let me be very clear that they shouldn't do things outside the European Union that are the property of the European Union.
"Why are we in the organisation? Why are we there in the first place?
"We are there because we are a trading nation and we want access to the single market and a full say about the rules of the single market, and what we can't have is the single market being discussed outside of the European Union and we will do everything possible to make sure that doesn't happen."
Mr Cameron insisted the safeguards the UK sought last month were "moderate, reasonable and relevant", and had been talked through with Ms Merkel.
But he said the French and German proposals were handed over "very late in the day".
In comments which put him at odds with Deputy Prime Minister Nick Clegg, Mr Cameron added: "We absolutely envisaged either a situation where we had a treaty change at 27 with safeguards, or Britain saying no to a European treaty.
"In those circumstances we were absolutely clear that it was very likely that other European countries in the euro, and some outside, would go ahead and sign a treaty outside the European Union and that is what it looks like is going to happen.
"As I say, we don't know how many countries will sign up to that treaty but Britain won't be one of them."