An investigation into a contract awarded to German manufacturer Siemens to provide trains to Channel Tunnel rail operator Eurostar has been opened by the European Commission.
The contract is being challenged by the firm's French rival Alstom.
It is reported that the complaint from Alstom concerns whether Eurostar violated laws regarding fairness in the award of a public contract.
Alstom failed to stop the deal last week in a UK High Court case.
The EU is awaiting a response from UK authorities to a letter sent from the office of the internal markets commissioner Michel Barnier before taking further steps.