It could be the bravest-ever act of military heroism.

Put yourself in the RAF boots of Sergeant Norman Jackson, a flight engineer in a Lancaster bomber detailed to attack Schweinfurt on the night of April 26, 1944. You are 20,000 feet in the air, travelling at 200 miles per hour over Germany. And you have just seen that the bomber's wing is on fire. What do you do?

What Sgt Jackson did was pick up a fire extinguisher, crawl out of the cockpit, back down the fuselage and onto the blazing wing. He was attached to the aircraft only by the cords of his parachute, which were in the hands of other crew members inside.

Suddenly the plane came under renewed fighter attack and banked sharply. Sgt Jackson slipped, fell through the flames and back behind the aircraft, hanging from the ropes of his parachute. The parachute then fell out but had been so damaged by the flames that it did not open properly.

Sgt Jackson plummeted towards the ground but landed on a bush, which saved his life. He sustained ankle injuries to add to the shrapnel wounds he suffered during the dogfight in the sky and spent the rest of the war in the Dalag Luft prisoner-of-war camp.

Sgt Jackson earned the Victoria Cross (VC) medal for his unbelievable courage, which could not save his bomber. He died ten years ago, aged 74. Now his VC is to be sold by auctioneer Spinks following the death of his widow and is expected to fetch more than £100,000 alongside his other medals.

The RAF Museum in Hendon, which already has 12 VCs in its collection, has been mentioned as a likely bidder for the medal, but it does not comment on potential acquisitions in case this drives up the price.

Henry Hall, director of the curatorial collection at museum in Grahame Park Way, said the medal was particularly interesting because of the story behind it.

"It's an extraordinary story but it's one of so many in the RAF, not in terms of VCs, but in individual achievements. Sgt Jackson didn't succeed in achieving anything in this particular venture but nonetheless it was a bold venture," he said.

"I don't think I would have done it I tried to imagine it when I first heard about it. It seems so extraordinary but clearly when the chips are down then you say I'll do it' to give my mates a chance.

"A more typical VC would be for pressing on in an attack having sustained damage that would justify turning back."

After the war, Sgt Jackson worked for a whisky company and is said not to have talked about the war often. In October 1945, he received his VC, an achievement that prompted a glowing front page tribute from the day's London Gazette newspaper: "It was an act of outstanding gallantry. Had he succeeded in subduing the flames, there was little or no prospect of his regaining the cockpit. By his ready willingness to face these dangers, he set an example of self-sacrifice which will ever be remembered."

Mr Hall said the RAF Museum has hundreds of medals of all types in its collection, including one VC that had been donated to it.