In an annual progress report, the EU executive said the two Balkan states must act urgently to eliminate high-level graft, beef up controls of their porous borders, improve food hygiene standards and strengthen their administrations and courts.
They should also speedily set up institutions to pay out billions of euros in EU aid, which the countries need to upgrade economies impoverished by decades of communist rule before 1989.
"Bulgaria and Romania have achieved significant progress so far in the preparations for accession. But the jury is still out," said Enlargement Commissioner Olli Rehn.
The Commission will recommend in April or May next year if the 1-year delay in accession is needed. The possibility of postponement is envisaged in the accession treaty that Bulgaria and Romania signed in April.
The two countries vowed to step up preparations for EU membership, saying they would do all they could to join the 25-nation bloc in January 1, 2007.
"I ask every prosecutor, every judge, every policeman to understand that they are part of a system which has started to work, and a failure of the justice system could translate into a major failure of the Romanian nation," Romanian President Traian Basescu told reporters.
Romania's Foreign Minister Mihai-Razvan Ungureanu warned of a "dangerous vicious circle" if Bucharest's EU accession were pushed back to 2008.
"Opponents of the EU in our country could make it more difficult to push through needed reforms," he said in a preview of an interview to appear in the German weekly magazine Stern.
Bulgarian Foreign Minister Ivailo Kalfin said: "Bulgaria will be able to do what is expected from it."
Rehn told the European Parliament in Strasbourg there remained a limited number of serious concerns, of which the main worry was corruption. Unless tackled, it would threaten the EU's internal market and EU-funded programmes in both countries.
The Commission is also asking Bulgaria and Romania to speed up reforms in the areas of human trafficking, counterfeiting, intellectual property protection and veterinary standards.
Under the accession treaty, the EU can also exclude one or both countries from some of the bloc's policies for a period.
For example, if a country has inadequate meat hygiene standards, the EU might ban imports from there for some time.