British Airways promised Thursday to punish those responsible for letting a jumbo jet fly with explosives left on board by a bomb-detection team during a drill.

The incident adds to the controversy in Britain over airline security which began with the Dec. 21 bombing of Pan Am Flight 103.A cleaning worker found the gelignite, which officials said could flare up but not explode, aboard the Boeing 747 on Monday at London's Heathrow Airport, said Independent Television News, which broke the story Wednesday night.

The flight had just returned from a flight to Toronto.

Police confirmed the incident, saying one of their bomb-detection teams inadvertently left the explosives on board after using the aircraft in an exercise to train sniffer dogs.

The Department of Transport said Thursday it has called for an urgent report. British Airways chairman Lord King promised "a most thorough and detailed investigation," saying: "If heads fall, they fall."

An airline spokesman, Derek Ross, said cleaners found the substance under a seat at the rear of the plane. ITN news said it was left there for about two weeks, during which the Boeing 747 made several journeys.

Airport sources said it was believed the package was placed in a seat pocket and then slipped through into the upholstery.

The latest controversy follows widespread charges that Tranpsort Secretary Paul Channon was lax about sending airlines warnings about a new type of radio cassette bomb before the Pan Am Flight 103 disaster on Dec. 21 that killed 270 people.

Flight 103, which originated in Frankfurt, was blown up over Lockerbie, Scotland, after taking off from Heathrow. Authorities believe a bomb was encased in a radio-cassette player.

Prime Minister Margaret Thatcher has defended Channon, but speculation remains that she will drop him from the Cabinet later this year.

Scotland Yard said police use British Airways planes for explosive-search training from time to time.

Police Inspector Andrew Nielson said the explosive weighed less than a pound and measured about 4 inches by .8 inch.

He said the substance was incapable of exploding by itself and "at the very worst it could flare up if ignited by fire. Nobody at any time has been placed in any danger."