The EU Commission president has called for a pause in EU enlargement after Bulgaria and Romania join the bloc.
Mr Barroso said he wanted Croatia "to join as quickly as possible"
Jose Manuel Barroso said the EU must first carry out institutional reforms and resolve the constitution problem.
His comments raise doubts about when Croatia and Turkey might join the EU. Both started entry talks in 2005.
The European Commission is expected to say on Tuesday that Bulgaria and Romania can join the 25-member bloc on 1 January 2007.
"There is no formal decision but... I think it would be unwise to bring in other member states apart from Bulgaria and Romania, which will be joining us soon, before we have sorted out the institutional question," Mr Barroso told reporters in Brussels.
"I do think it would not be wise to proceed with any enlargements before we have resolved the constitutional issue in Europe," he said.
Mr Barroso stressed that there were limits to the EU's capacity to absorb new members without new rules to make an expanded bloc work effectively.
However, he said he "would like Croatia to join as quickly as possible, if it fulfils all the criteria".
Croatia has expressed hopes of completing its membership negotiations by 2009.
Mr Barroso made no mention of the other candidates for membership - Turkey and the Former Yugoslav Republic of Macedonia, which officially gained candidate status in 2005.
A current EU treaty provides for a maximum of 27 members before institutional changes are needed.
Plans for the bloc to adopt its first constitution suffered a setback last year when Dutch and French voters rejected the charter.
The European Commission is expected to confirm on Tuesday that Bulgaria and Romania can join the bloc in January 2007, rather than in 2008, the BBC's Oana Lungescu in Brussels says.
But, according to a draft report seen by the BBC, both countries may face sanctions without deep reforms, our correspondent says.
Romania and Bulgaria risk food export bans and cuts to EU funds, the draft document says.
They will also be checked on corruption and judicial reform in some of the EU's toughest ever entry conditions.
The BBC's Jonny Dymond in Brussels says that without a new way of taking decisions, it is difficult to see how further enlargement could take place.
But given the nervousness about such enlargement, many will interpret Mr Barroso's comments as another sign of the EU's unwillingness to take on new members, he says.