There is no doubt it's a full-scale emergency

RAF spokesman

Commenting on the decision to send the helicopters he said: "At the moment they are getting into positions where they can lift the passengers off if necessary and put them onto nearby oil rigs.

"There is no doubt it's a full-scale emergency, the biggest mobilisation anyone can remember for a long, long time."

Further helicopters attached to the oil rigs have also been put on alert in case it is decided to evacuate the ship, 140 miles off the Northumberland coast.

The sea is said to be relatively calm, with some fog.

Anyone anxious about relatives on board the vessel should telephone 01255 244 377.

Communications difficult

Mark Clark, a spokesman for the Maritime and Coastguard Agency, told BBC News 24 the mood among the passengers was not yet known as communications with the ship were difficult.

RAF helicopters are ready to help those on board

He said: "At the time it was reported the Master decided that his own fire fighting equipment could handle the blaze.

"Initially he reported an engine room fire, which is far more serious. We obviously know they have a considerable amount of fire fighting equipment but we are standing by if the order is given to abandon ship."

Mr Clark said whether they should be evacuated was the master's decision and if he gave the go-ahead the operation would take a "matter of minutes".

If an evacuation does go ahead the rescue aircraft are expected to take passengers and non-essential crew to Newcastle-upon-Tyne.

Mr Clark's colleague, Michael Coull, said the vessel would probably be brought into a Scottish port.

It no longer has the use of its engines and the options for moving it are being assessed.

Large fleet

The ferry is operated by DFDS Seaways, part of the Danish DFDS group, which has one of the largest fleets in northern Europe.

It runs between Newcastle, Kristiansand in Norway and Gothenburg in Sweden.

The ship was built in 1976 and refurbished in 199.

It is 184 metres long and weighs 22,528 tonnes.

It has a sister ship, the Prince of Scandinavia.

The RAF Nimrod was sent from RAF Kinloss and the helicopters from Lossiemouth, Boulmer and Wattisham.

They were being joined by a Royal Navy helicopter from Prestwick, Ayrshire, two oil rig helicopters and a Norwegian search and rescue helicopter.