David Cameron has 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. 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: "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: "Let me be very clear that they shouldn't do things outside the European Union that are the property of the European Union."
Explaining why the UK is in the organisation, he said: "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".
Pressed on what safeguards he had secured for the UK, Mr Cameron said: "What I stopped was that if you have a treaty within the framework of the European Union that didn't have safeguards on the single market and on financial services, Britain would have been in a worse position.
"I am not making some great claim to have achieved a safeguard but what I did do was stop a treaty without safeguards. Is that clear enough?"