The European Union's top official in charge of its enlargement Friday warned Bulgaria and Romania that their hopes of joining the bloc in 2007 could be delayed if they do not fulfil accession commitments.

Enlargement Commissioner Olli Rehn's comments came days after the European Union delayed the start of accession talks with Croatia over Zagreb's failure to locate an indicted war crimes suspect.

Speaking to the Bulgarian parliament, Rehn pointed to a safeguard clause that has been inserted into membership negotiations with both countries.

It "allows member states based on a proposal by the European Commission to postpone the planned date of accession by one or both countries by one year," he said.

"This will occur in the case where the country is manifestly unprepared in an important number of areas."

In such a case, Rehn said, the commission "will not hesitate to make use of these remedial measures" to defend the interests of existing member states.

"I very much want to be in a position that I do not have to recommend the use of the safeguard clause. But that is in the hands of the countries," Rehn explained.

Rehn's warning is not new -- the European Union has already indicated that membership could be delayed if Sofia and Bucharest do not make enough progress on areas such as tackling corruption and reforming their justice systems.

The reform of the judiciary to make it "more efficient, reliable, effective and transparent" is "an overreaching challenge" for Bulgaria. "This is the priority of priorities," Rehn said.

"This reform is indispensable to ensure that Bulgaria has the legal means and resources to apply fully the rule of law and prosecute in a satisfactory manner organised crime and corruption," he added.

The EU commissioner also pointed to the necessity that already designed protection schemes for vulnerable citizens, and especially the Roma, "will translate shortly into concrete actions and measures" that will improve their position in society.

Concerning Romania, which has attracted much of the critical focus of the EU, Rehn stressed the speedy progress being made recently, especially since last December when the Dutch presidency introduced the safeguard clause into the text of the accession treaty.

"If reinforced efforts continue and have support in the Romanian parliament then Romania has a fair chance of meeting the criteria and becoming a member in 2007," Rehn added.

Apart from tackling the judiciary reform and the fight against corruption, Romania has also been ordered to implement reforms in domestic affairs and market competition.

Both countries are due to sign formal accession treaties with the European Union on April 27 with a view to joining the bloc on January 1, 2007.

Earlier this week the European Union refused to start talks with Croatia as planned after finding that Zagreb had not cooperated sufficiently with the UN war crimes tribunal for the former Yugoslavia.

It follows the failure to locate Ante Gotovina, a retired general indicted by the tribunal in The Hague for the alleged murder of ethnic Serbs during the final stages of Croatia's 1991-1995 war.

EU leaders told Zagreb that the door to membership would be opened as soon as it provides full cooperation.