Tony Blair arriving in Brussels for a marathon round of talks on Europe

Speaking in Brussels, Mr Blair conceded that "it may well not be possible" to strike a deal, adding that the various negotiating points were way apart.

"It's important for us to get the right agreement, not simply any agreement," he told reporters.

The talks follow a deal hammered out on Thursday on European defence.

Peacebroker?

EU leaders are debating the way the EU constitution should look once 10 new member states join the union next year, expanding its membership to 25.

A row over national voting power in EU meetings has become a real stumbling block, with Germany and France on one side and Spain and Portugal on the other.

It is going to be very, very difficult because the positions are a long way apart Tony Blair

Mr Blair has been seen as a possible peacebroker, especially after a "constructive" private breakfast meeting with German Chancellor Gerhard Schroeder and French President Jacques Chirac before the summit began.

But on Friday, Mr Blair did not underestimate the task ahead.

"What's important is to see if we can get an agreement that satisfies the interests of all countries - but it's going to be very, very difficult," he told reporters.

Asked how many shirts the prime minister had brought with him for the duration of his stay, he replied: "A lot."

Jobs

He stressed: "The positions are a long way apart.

"There's only a point in having an agreement if that agreement makes the European Union workable and effective on the issues that people really care about."

Blair's red lines No EU force which threatens NATO No removal of national veto Taxation to be decided by states alone Individual legal systems to be maintained Votes on social security should be unanimous Budget changes to be unanimous Deal on EU defence Poland may veto constitution

He said the issues were not the intricacies of some of the constitutional questions, "but questions to do with jobs ... growth in Europe [and] to do with protecting ourselves on issues like security and terrorism".

A deal was important because the EU was expanding from 15 nations to 25. "It's going to be the biggest economic market in the world," said Mr Blair.

"Britain's vital interests are engaged in making sure, therefore, that this new EU works effectively."

Mr Blair arrived in Brussels armed with the "red line" issues on national sovereignty over which he has vowed the UK will not cross.

No more 'superstate'

On the table is how to retain the balance of power between Europe's big players, including the UK, and smaller nations who fear they will lose influence when the EU expands next year.

Foreign Secretary Jack Straw also admitted the talks would be tough, but stressed that Britain would not be putting its signature to anything that aspired to a European superstate.

I do like placing a bet, but I'm not placing any money with any bookie on when or whether this Inter Governmental Conference is going to finish Jack Straw

"Any draft that we sign up to will not further deepen the European Union, it will simply be there to make it more efficient and it will actually mark the end of any aspirations for a European federal superstate - that's yesterday's fantasy, that's not tomorrow's reality.

"The phrase 'ever closer union' has been dropped from the constitution - it's no longer an explicit aspiration which has been actually in the constitution, in the treaties of the European Union, for the last 50 years."

But Mr Straw told BBC Radio 4's Today programme he would not be putting any money on the chances of a deal being struck this weekend.

"I think it's a matter with the Almighty," he said.

"I do like placing a bet, but I'm not placing any money with any bookie on when or whether this conference is going to finish."

Defence

The constitution will be a blueprint for how the EU is run in the future and will replace existing treaties.

Britain is keen to retain its sovereignty on key issues like keeping vetoes on foreign policy and avoiding tax harmonisation.

The countries have managed to agree on having their own defence operations, separate but complementary to Nato.

The proposals, understood to have US backing, will include the ability to plan and run "certain operations".