A plane flying from Europe to the US has been diverted after a cryptic note hinting at a bomb was reportedly found scrawled on the wall in the aircraft's bathroom.

The airline has confirmed that the reason for the unexpected diversion to Shannon airport was due to a "potential security concern".

Passengers on board United Flight 971 have disembarked from the plane and both checked and hand luggage is being re-screened.

The crew declared a May Day at around 12.45pm and informed air traffic controllers that they wished to divert to Shannon Airport.

Shannon airport implemented emergency procedures which involved alerting the local authority fire service, HSE National Ambulance Service and An Garda Síochána.

The flight's 214 passengers were removed from the plane, and sent to a nearby hotel.

"We will work to get customers on their way to Chicago as quickly as possible," United said in a statement.

According to the Irish Times, gardai are taking handwriting samples from passengers to try and establish who is behind the note.

This story is developing, stay with us for more information.