The European Commission said it will investigate the public financing for the Fehmarn Belt link to see if it in line with state aid rules.
The announcement follows the European Union General Court's annulment of a previous Commission decision approving the support.
"Today's opening of such an investigation is an invitation for all stakeholders to provide their input, which will allow the Commission to adopt a new, well-informed final decision,” said Margrethe Vestager, EU Competition Commissioner.