The story has been updated on the Sky News website, link same as opening thread;
quote "A Sea King helicopter and lifeboats have been scrambled in a rescue operation.
A pilot flying over the spot earlier today saw a stricken aircraft on the surface of the water.
When he returned for a second pass he could see no sign of it, or of any life rafts or debris.
However, he put out a "mayday" call and a rescue operation is now underway.
Michael Mulford, of RAF Search and Rescue, told Sky News: "There's always the possibility of an innocent explanation.
"At the moment it's very much a question of get a helicopter there, get some lifeboats there and see if there's anything there."
He added there were no reports of any aircraft missing, but traffic was now being monitored.
Ships in the area are also being asked to look out for any sign of an aircraft."
"Our 319's are very reliable. They get fixed very quickly."