A team of swimmers has been rescued from a support boat after it sank during the annual Perth to Rottnest swim event.

The boat was about two nautical miles from Cottesloe when it issued a mayday call.

A race official said the boat had begun taking water and sank.

Four swimmers were picked up by nearby boats.

It is believed the boat’s bilge pump failed.

The stricken boat was being towed back to Fremantle, the official said.

The event got underway at 5:45am when Premier Colin Barnett fired the starter gun at Cottesloe beach.

A total of 2400 swimmers are competing this year in the gruelling 19.7km crossing.