An unexplained “security incident” and Royal Air Force operation closed airspace around Heathrow on Friday morning, with dozens of arriving planes forced to stack for an hour or divert to other airports due to lack of fuel.

A military flight from nearby RAF Northolt stopped scheduled passenger planes landing at Heathrow, Britain’s busiest airport, for about 20 minutes from 9am.

Military and civilian VIP aircraft use RAF Northolt, near Ruislip in west London, home to units from the armed services and the Ministry of Defence. The RAF’s 32 Squadron, which operates flights for the royal family, is based at Northolt.

A spokesman for Heathrow said the RAF’s request to close the airspace was “unplanned”. He said: “Arrivals were paused briefly this morning due to an RAF request for an operational flight within part of Heathrow’s airspace.”

The RAF said the flight had taken longer than anticipated. A spokesperson said: “The RAF can confirm that a flight was completed this morning by one of our assets from RAF Northolt.

“This flight was coordinated with Heathrow ATC but had to extend by a few minutes to complete its sortie. The minor delays caused to civilian air traffic are regretted.”

At least four British Airways flights and one Virgin Atlantic flight were forced to divert to other London airports including Gatwick, Stansted and Luton.

Passengers on affected planes said they were told by pilots that the delay was due to a security incident.

One passenger on a Virgin Atlantic flight from Los Angeles that diverted to Gatwick shortly before it was due to land spoke of the “alarming” experience as the plane circled Heathrow.

Anne Henry, a British TV producer, told the Mirror Online: “It was quite alarming when the captain announced after circling Heathrow for a while: ‘Ladies and gentlemen, we have about five to 10 minutes’ fuel left so we may need to divert to Gatwick.’”

After the Virgin plane refuelled, it took off again for Heathrow and landed almost three hours late. Passengers were given a letter apologising for a “security incident” disrupting the flight.

The air traffic control service, Nats, said: “We worked closely with Heathrow this morning to minimise disruption following the pause in operations. Services are now operating normally.”