Reports say a massive wave hit one of the legs of the platform, causing it to break and send the 208 people on board into the sea at around 1830 GMT.

Some were able to make it to the lifeboats before the platform fully capsized while others were thrown into the sea as the rig began to tilt.

Most of those missing from the Alexander Kielland platform, which was situated 235 miles east of Dundee, are Norwegian.

A spokesman for Phillips Petroleum, the American owners of the rig the platform was connected to, said: "The rig is now bottom up after capsizing completely and dropping people into the sea."

RAF and Norwegian helicopters have been sent to the area along with local ships who have been asked to help with the rescue.

An RAF Nimrod, complete with searchlights and flares is on its way to the scene.

However, poor weather conditions are making the rescue very difficult and there are reports of people being swept away as they attempt to reach rescue boats or neighbouring rigs.

The Norwegian Government is holding an emergency meeting to discuss the disaster.

The accommodation platform contains bedrooms, lounges, kitchens and leisure facilities for workers. It was attached to the Edda oil rig in the Ekofisk field where work was being carried out.

The platform is a semi-submersible that floats on two pontoons with legs supporting the main deck. It houses workers while they carry out jobs on the oil rigs.

This is the second major accident at Ekofisk field. A blow-out on the Bravo platform in 1977 caused a mass evacuation of all on-board.

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