There had been no oil spill from the tanker, he said, but in order to set it free, the vessel had been emptied of around 3,000 tonnes of the 35,000 tonnes of gasoil it had been carrying when it ran aground.

The Bahamas-flagged Acushnet had already been set free on Sunday but was grounded again on the same sand bank because water levels were too low for the vessel to sail.

The Acushnet, which was bound for the United States, is a single-hulled tanker like the Prestige, which sank off Spain in November spilling about one-third of its 77,000-tonne cargo of fuel oil and causing Spain's worst environmental disaster.

It remained unclear why the Acushnet had run aground in the channel between the Jutland peninsula and Sealand island - a stretch of water notorious for shipping accidents.

Acushnet had chosen not to use a pilot while passing through channel - a decision that could have caused it to veer off course on its journey from Ventspils in Latvia to Boston, the spokesman said.