The alert came shortly after take-off from Heathrow

The aircraft was diverted after a crew member found a "troubling" written note on board, an airline official said.

The Boeing 777 landed safely at Shannon Airport in south west Ireland, where officials said a full security alert took place.

All passengers on board were removed and searched together with crew members, an airline official said.

'Priority'

The plane has now been parked on a remote spot at the airfield.

The note was found in a seatback pocket by one of the crew, said an airline spokeswoman.

United Airlines spokeswoman Chris Nardella declined to release any details of what was written in the note, but said the plane was diverted because "safety and security is our main priority".

The plane, carrying approximately 196 passengers and crew had only just taken off from London's Heathrow Airport when the note was found.

Spokeswoman for Aer Rianta, which runs Ireland's airports, said Shannon Airport authorities had ordered the aircraft to be parked, given the suspicions that a bomb could be on board.

Searched

But she said regularly scheduled flights were continuing to land and take off from Shannon.

Jim Malloy, a spokesman for the Garda Siochana, Ireland's national police force, said: "Everybody aboard was taken to a secure area, and obviously they were all searched, but nothing suspicious was found."

Ms Nardella said the passengers will be kept at Shannon until the airline decides whether it is appropriate to fly the plane on to the United States or arranges an alternate means of travel.

The airplane was due to land at New York's John F Kennedy Airport at 1610pm (2210BST)