A two-inch door to the fuel tank of the aircraft was found unscrewed

Air accident investigators revealed the pilot had to declare an emergency, and released most of the plane's fuel over the sea, before returning to land.

The incident took place on a Boeing 777 airliner, which had 151 passengers on board, on 10 June.

Investigators probing the failure described it as "serious".

We will continue to co-operate with the full AAIB investigation into this incident, which is not yet complete BA spokesperson

The airline was heading towards Windsor when another pilot, waiting to depart, noticed the trail of smoke and the smell of aviation fuel.

The British Airways crew believed the plane was leaking fuel and immediately informed air traffic control of an emergency.

The plane released 90,000 kilos of fuel to enable it to make a safe landing back at Heathrow.

A two-inch door to the fuel tank was later found unscrewed. The screws were found hanging in a small plastic bag.

British Airways says it is co-operating fully with the investigation and that it has undertaken its own inquiry.

A BA spokesperson said: "Safety and security is always our first priority and following our own investigation we immediately put in place a number of procedures to prevent this incident recurring.

'Continue to co-operate'

"We have amended our own fuel tank maintenance procedures and are working with the aircraft manufacturers to ensure that they amend the aircraft maintenance manual accordingly."

The details of the incident have been published in a special bulletin by the Air Accidents Investigation Branch (AAIB).

"We will continue to co-operate with the full AAIB investigation into this incident which is not yet complete and are committed to ensuring any further recommendations are implemented accordingly," the spokesperson said.

The bulletin says the plane was scheduled to fly to Harare, in Zimbabwe.