European Commissioner for Competition Margrethe Vestager | Frederick Florin/AFP via Getty Images Margrethe Vestager escalates ports dispute with France and Belgium Ports of Antwerp and Le Havre among those suspected of benefiting from illegal tax breaks.

France and Belgium should stop giving special tax breaks to their sea and inland ports because it gives them an unfair advantage over ports in other countries, the European Commission said Friday as it opened a formal state-aid probe.

The launch of the investigation escalates a dispute that started in January, when European regulators first asked Paris and Brussels to tax their ports in line with other economic sectors.

"Tax exemptions shouldn't distort competition by giving an unfair advantage to some ports over others in Europe," said Margrethe Vestager, the European commissioner for competition.

The Commission named Antwerp and Le Havre, Europe's 2nd- and 11th-busiest ports, as being among the beneficiaries.

Europe's ports and ship builders have come under intense scrutiny from regulators in recent years, as Brussels seeks to encourage a level-playing field and force ports to adapt to global competition.

Vestager has also borne down on national capitals using tax breaks to channel illegal aid to favorite companies.

The in-depth inquiry will establish whether "initial concerns are confirmed or not," said the Commission, adding that none of the ports would be liable to pay back any state aid that it subsequently deems illegal, given that their special tax status dates back to before the creation of the European Union.