David Cameron was accused by Labour of compromising British interests on Friday after he admitted he had not achieved a "safeguard" for the UK when he vetoed an EU treaty in December.
At the time of the historic veto David Cameron claimed to have acted to protect the City of London, but many economic commentators have since claimed that failing to engage with other European leaders could lead Britain's financial hub sidelined.
On Friday morning the BBC journalist Evan Davis challenged the PM on this, and Cameron replied: "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?"
Labour seized on the remarks, with shadow foreign secretary Douglas Alexander saying he had "admitted that he secured no safeguards for the UK".
"Giving up on negotiations has compromised British interests not safeguarded them and means the UK will struggle to have a voice in talks that could have a profound effect on jobs and growth in Britain."
While the prime minister promised he would try and do "everything possible" to prevent the countries who had signed up to December's treaty using EU buildings, he was unclear if this could be achieved.
"Part of the problem is that the legal position is unclear," he told the BBC.
"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."
Reacting, UKIP leader Nigel Farage said the use of "the so-called veto" was "no more than a fudge".
"David Cameron has finally admitted that he has not achieved some great safeguard for British business. Nothing has changed."