The EU, the United States and human rights organisations have accused Prime Minister Viktor Orbán of using constitutional amendments to limit the powers of Hungary's top court and undermine democracy in the former Soviet satellite.
In a letter to Orbán on Friday (12 April), European Commission President José Manuel Barroso said that, based on a first legal analysis, Brussels had serious concerns over the compatibility of the constitutional changes with EU legislation and the rule of law.
Once it has completed its legal analysis, the Commission "will have to take the necessary steps in order to start infringement procedures where relevant," Barroso told Orbán.
He appealed to the prime minister, the leader of the nationalist Fidesz party, to "address these concerns and to tackle them in a determined and unambiguous way."
Orbán has dismissed criticism that the constitutional changes are anti-democratic and last month challenged EU legal experts to present evidence if they had any problems.
EU officials see the changes as part of a series of steps taken by Orbán's government that they say have undermined democratic accountability. Other measures include restrictions on press freedom and the enforced retirement of judges.