Nick Clegg says it would be ludicrous to create new institutions, but No 10 says it may try to block eurozone countries from using existing bodies
Fresh coalition splits have emerged as Downing Street indicated it may block the euro-group countries from using EU institutions – including its buildings – to plan co-ordination of the eurozone.
A Number 10 spokesman also said UK demands for a level playing field for UK financial services remained the UK objective.
The deputy prime minister, Nick Clegg, in his interview with Andrew Marr on Sunday, said: "Well it would be ludicrous for the 26, which is pretty well the whole of the European Union with the exception of only one member state, to completely reinvent or recreate a whole panoply of new institutions."
He added the euro-group member states "had already said that there's going to be some crossover between institutions that we all share".
Clegg appears to be eager to use the future discussions about the inter-governmental agreement, and its relationship with the EU institutions, such as the European Court of Justice, to try to undo what he regards as the damage of Friday's talks.
But the prime minister's spokesman indicated Britain was going to make sure that nothing happened that meant the existing treaty was cut across or rolled back by allowing the 17 to use EU institutions.
He said "we are in uncharted territory", adding there was "a potential conflict of interest" if the EU institutions were serving two masters, the 17 strong euro-group and the 27 members of the EU at the same time.
He added: "They made a decision last week to push ahead with an inter-governmental agreement, and this is a 'treaty within a treaty'. He said discussions about how this inter-governmental agreement will be implemented had just started.
"There is a discussion going on in Brussels about how this inter-governmental agreement should be implemented, and we will be involved in these discussions. We would be concerned about confusion of roles, conflicts of interests, and we need to explore these."
He said one of the issues was whether the 17 could use the EU buildings in Brussels to hold its discussions.
He insisted Britain had not been seeking a carve-out from EU financial services directives, but merely a level playing field, and said Britain would continue to have that objective, but these discussions may take place in a different forum and a different context in Ecofin.
"We were looking to see whether we could use this proposal for a treaty of 27 to deal with some of those issues, but we will continue to pursue the same objectives," the prime minister's spokesman said.
He admitted there were disagreements within the coalition on Europe, but said David Cameron had taken Nick Clegg through the UK negotiating strategy before he went to Brussels.
He also insisted that Cameron had made it clear in public and private that he would wield the veto if he did not secure his objectives in the talks.
He said Cameron had contacted both Clegg and the chancellor, George Osborne, in the early hours to discuss the outcome of the talks.