A flight crew's reluctance to use cup holders in the cockpit caused a passenger plane to be turned around midway across the Atlantic after coffee was spilled over the aircraft's radio controls, a report has found.

A 49-year-old pilot was taking 326 passengers from Frankfurt in Germany to Cancun in Mexico on February 6 this year.

At 4.20pm, when the aircraft was west of Ireland over the North Atlantic Ocean, the pilot put his coffee on a tray table as he carried out checks.

The Air Accident Investigations Branch (AAIB) stated in its report that crew did not like using the holders supplied because the cups were too small, making it difficult to get them in or out.

As a result, the cup, containing the hot beverage, was knocked into the pilot's lap and some liquid splashed onto the control panel.

It immediately caused the public address announcements system to fail, but 40 minutes later the unit became very hot and a burning electrical smell started to fill the cockpit.

An hour after the initial spillage, the unit had failed and became so hot that one of the buttons melted, but by this stage the flight was almost half way across the Atlantic.

The pilot, with more than 13,000 hours of experience, decided he had to turn around and diverted to Shannon in the Republic of Ireland.