The 25 leaders of the European Union countries have begun a summit in Brussels aimed at hammering out a future direction for the EU.
Leaders must decide whether to revive the constitution
They are considering how it should be organised and how much it should grow.
They are expected to extend a year-long period of reflection on what to do with the EU constitution after its rejection by France and the Netherlands in 2005.
Austria is also pushing for a decision that could further slow down the bloc's enlargement to the south and east.
EU countries are split between those who would like to bury the constitution, and those who would like it to be revived, in one form or another.
At this summit, some want to agree on 2008 or 2009 as a target date for institutional changes of the kind mapped out in the constitution, but others want to avoid setting a timetable.
The Polish Prime Minister, Kazimierz Marcinkiewicz, made clear he favoured an open open-ended pause.
"I am convinced that at this summit, we will simply extend the period of reflection. We will try not to prejudge how long this extension should be," he told reporters.
British Foreign Secretary Margaret Beckett said there seemed to be less consensus now on the way forward than there had been before.
Delicate talks will be needed on issues facing the EU
"So an extended period of reflection would seem to make sense," she said.
The debate on enlargement is expected to turn on the question of how much weight to give to the EU's capacity to "absorb" new members.
The EU has always said that this is an "important consideration" affecting decisions about new members, but there is now a move from Austria, the outgoing EU president, to make the language stronger.
BBC Europe Editor Mark Mardell says some countries want further enlargement to be conditional on the support of public opinion in the EU.
The President of the European Commission, Jose Manuel Barroso, told the BBC in an interview that all democracies had to take account of public opinion. He added that it was possible for Turkey to join the EU, but it would be very difficult.
"First of all (it will be) very demanding for them but also demanding for us here to be ready to accommodate such an important big country that is seen by so many of us as culturally different from let's say mainstream Europe," he said.
France and Germany are backing Austria on enlargement, but the UK and the newer member states argue that erecting new barriers would be unfair to potential candidates.
Other topics which may arise include:
- Transparency - Austria says discussions of EU legislation by the Council of Ministers should be televised. The UK, which took the same view during its presidency last year, has made a U-turn, and is now opposing the idea
- Foreign policy - the European Commission has proposed closer co-operation between the EU's foreign policy and security chief, Javier Solana, who answers to member states, and the European Commission's external relations apparatus
- Co-operation on crime - the Commission wants member states to give up their veto in the area of criminal justice, on the grounds that this would help produce common policies on cross-border crime and terrorism. The UK and France are in favour, but Germany is not.