European Union foreign ministers are holding fresh talks aimed at reaching agreement on a new constitution for the enlarged bloc of 25 countries.
Jack Straw does not expect an overall agreement until June
Last December an attempt to reach consensus broke down because of objections from Spain and Poland over voting rights.
The ministers are expecting two days of hard bargaining in Brussels.
UK Foreign Secretary Jack Straw warned there was no prospect of an overall agreement, at least until June.
He is expected to demand that vetoes on taxation, foreign policy and defence remain.
Before the meeting, he said: "We're going in with a very constructive and positive approach, but our views are clear, they're well known, and I dare say there'll be a couple of days of hard bargaining ahead.
"There is no prospect of overall agreement, at least until the June European Council."
The UK has some allies, but there are others who argue that too many vetoes would make it impossible for the EU to work properly.
The BBC's Europe correspondent Chris Morris says a lot of work is being done behind the scenes to find a new formula for voting on future EU decisions.
There is also a proposal that there should be no reduction in the size of the European Commission until 2014, allowing each of the 25 member states to appoint a commissioner for the next 10 years.
There are widespread fears that the enlarged EU could get bogged down in disputes on a range of issues.
Irish Prime Minister Bertie Ahern - whose country currently holds the EU presidency - hopes the negotiations will go sufficiently smoothly to sign a deal at a summit he will host in Brussels in mid-June.
Since the ministers last discuss the constitution, the UK government has changed tack and announced that it will hold a public referendum on the treaty.
That could be the biggest obstacle of all, even if EU governments do manage to agree among themselves in a month's time, our correspondent says.
Last week UK Chancellor Gordon Brown said the UK government would not back down over issues affecting sovereignty.
Some governments - and European Commission President Romano Prodi - say the principle of unanimity in an EU of 25 states will not be practical.
Mr Ahern is keen to win a deal before the current Irish EU presidency expires at the end of June.
But Belgian Foreign Minister Louis Michel believes European Parliament elections between 10 and 13 June could limit governments' room for manoeuvre.
"I don't know if it will be possible for certain countries to relax their positions before the European elections," he said.
No decisions will be taken in Brussels this week, but the meeting of foreign ministers should make it clearer whether differences on the constitution details can be resolved in the next four weeks.