The incident happened on the Beryl Bravo platform, 185 miles north north east of Aberdeen at 0300 BST.

The crew members were taken to hospital in Shetland suffering minor injuries.

The release occurred during routine maintenance associated with a planned platform shutdown.

A spokesman for Aberdeen Coastguard said they were flown to Shetland as a "precautionary measure".

Investigation begins

A spokeswoman for ExxonMobil, which owns the platform, said three of the men taken to the Gilbert Bain Hospital in Lerwick had since been released.

The two other casualties were being kept in hospital overnight for observation but their injuries were not thought to be life-threatening.

A total of 235 personnel were on the platform at the time of the incident.

The spokeswoman said an investigation was under way to find out the cause of the gas leak.