The European Union is investigating tax breaks given to ports in France and Belgium to see whether they comply with state aid rules or give the ports an edge over competitors.

The EU's anti-trust watchdog asked France and Belgium in January to bring their corporate tax law into line with Europe's rules by abolishing a tax exemption for ports, but the two countries have not done so.

Most of France's ports are exempt from corporate tax, while many of Belgium's big ports are subject to lower rates than other companies.

Competition Commissioner Margrethe Vestager said Friday that "tax exemptions shouldn't distort competition by giving an unfair advantage to some ports over others in Europe."

France and Belgium could face fines if they fail to comply with EU rules.