Schengen migrants: EU concern at Danish border plan
The EU has voiced concerns over Denmark's plans to reintroduce controls on internal borders, abolished under the Schengen Agreement.
European Commission chief Jose Manuel Barroso questioned whether the measures would be compatible with the principle of free movement.
Denmark has denied its plan will violate EU treaties.
Officials say they will maintain passport-free travel using customs patrols to fight crime.
EU interior ministers said on Thursday that the Schengen rules needed to be clarified to deter countries from unilaterally reimposing border checks.
They were reviewing the 25-nation agreement because of tensions over this year's influx of migrants from North Africa.
More than 25,000 Africans, most of them Tunisians, have arrived by boat in Italy since unrest began in the Arab world this year.
On Wednesday, Denmark announced that it would reinstate control booths on its borders with Germany and Sweden within weeks.
It said it would carry out random checks of cars and passports, deploying more customs officers and video surveillance to tackle cross-border crime.
In a letter to Danish Prime Minister Lars Lekke Rasmussen on Friday, Mr Barroso said an initial legal assessment of the move raised "important doubts about whether the proposed measures, if implemented in the 'intensive and permanent' way that has been announced, would be in line with Denmark's obligations under European and international law".
He referred in particular to the obligation to respect "free movement of goods, persons, services and capital, and the provisions of the Schengen Borders Code".
Cecilia Malmstrom, EU Commissioner for Home Affairs, said later that the commission had "real concerns" over the prospect of "a permanent and visible customs control at all Danish borders".
The commission, she said, would ask Denmark for information about the legal basis for its envisaged checks.
"I call on the Danish government to refrain from taking unilateral steps and to make sure that any measures taken are in line with the relevant law," she said.
"The commission stands ready to continue the dialogue with Denmark. But it will, if needed, use the tools at its disposal to guarantee the respect of EU law."
Schengen allows for the temporary reimposition of border controls in special cases to ensure public order but France and Italy say a migrant surge is also grounds for reintroducing such checks.
The European Commission says such measures should be "an absolute last resort".
Modifications to Schengen could be approved at a European summit next month.