http://news.sky.com/skynews/article/...,00.html?f=dta
A major sea rescue attempt is under way, a spokesman for the Maritime and Coastguard Agency said.
It is believed that a number of crew are missing. The area is being scoured by rescuers for survivors.
Michael Mulford, of the Royal Air Force Rescue Centre, said contact with the helicopter was lost at about 6pm and it was believed to have crashed in the water.
He told Sky News: "It is a major search and rescue operation at this point.
"We are looking for seven people who are almost certainly going to be in the water."
There were no immediate details on what type of helicopter it was or who was aboard.
Mr Mulford said rescue planes and boats had been dispatched to search for the wreckage and any survivors in the large bay just east of the Isle of Man.
He said: "It is generally calm and relatively quiet there. What we have are good searching conditions, albeit it is dark.
"It is bound to be very cold in the water at this point."
From the Beeb:-
The aircraft is said to have been ferrying personnel from the gas field in the Lancashire bay.
A search for survivors involving two RAF rescue helicopters, lifeboats, coastguard crews and a rig support vessel is under way.
The daughter of one rig worker said he had phoned his wife to say the helicopter crashed at about 1740 GMT.
The helicopter crashed in the Irish sea en route to the gas rig platform in the bay.
Two helicopter crew members and five rig workers are now thought to be missing.
Michael Mulford, from the Royal Air Force Rescue Centre, said contact with the helicopter was lost at around 6pm and it is believed to have crashed into the water.
He told Sky News: "We are looking for seven people who are almost certainly going to be in the water."
It is understood that the helicopter crashed 20 miles off the Lancashire coast.
Mr Mulford added: "What we have are good searching conditions, albeit it is dark. It is bound to be very cold in the water at this point."