Denmark has got EU approval for a 19km (12-mile) undersea road-rail tunnel that will cut journey times to Germany.
The €8.7bn (£6bn; $9.6bn) Fehmarn Belt project will get €589m of EU funding.
The EU Commission said it complied with EU state aid rules and would boost transport links between central Europe and Scandinavia.
The tunnel will connect the Danish island of Lolland with Germany's Fehmarn island. Construction will start in January, and it should open in 2024.
There will be a four-lane motorway alongside a double-track railway.
The link will provide a fast corridor between Copenhagen and Hamburg. It is expected to slash that rail journey from just under five hours currently to little more than two hours.
The Oresund road-rail bridge already connects Copenhagen with Malmo in southern Sweden.
The EU contribution to the Fehmarn tunnel will come from the Connecting Europe Facility, a scheme for modernising Europe's transport infrastructure.
In a statement on Thursday, the Commission said the new tunnel would be integrated with Denmark's public transport and it was "neither liable to distort competition nor to affect trade between member states".
The tunnel will be built by Denmark's state-owned Femern A/S.