Ronnie Russell, 72, bravely intervened and punched Ian Ball twice in the head during a brazen attempt to snatch the princess at gunpoint yards from Buckingham Palace in 1974.

The former heavyweight was later presented with the George Medal by Her Majesty, who told him: ‘The medal is from the Queen, but I want to thank you as Anne’s mother.’

Mr Russell, from Bristol, is now in poor health after suffering several strokes and is reluctantly auctioning off the medal, tipped to fetch up to £20,000 to provide for his future.

The have-a-go-hero said: ‘It was something I said I would never, ever do. I am so proud and honoured to have done such a thing and be involved in it.

‘Whoever does eventually buy the medal, I would hope they might invite me somewhere to tell them an after-dinner story about what happened.’

Ball used his Ford Escort to block off the Rolls-Royce carrying a 24-year-old Princess Anne and her then husband Capt Mark Phillips as it drove back to the Palace along Pall Mall.

He then fired a series of shots through the rear window. The pair were unhurt but three others, including the princess’ personal detective, were wounded.

Mr Russell, who was 6ft 2ins, 17st and had trained at the same gym as the Kray twins, was making his way home from his job as a cleaning manager when he came across the drama unfolding.

He said he spotted Ball trying to drag Princess Anne from her car while her husband was desperately trying to cling on to her.

The boxer said: ‘She was very, very together, telling him, “Just go away and don’t be such a silly man”.

‘He stood there glaring at me with the gun and I hit him. I hit him as hard as I could – if he had been a tree he would have fallen over – and he was flat on the floor face down.’

Mr Russell has since revealed that police paid him a visit before he received his medal.

He told the Daily Mirror: ‘They were looking round my home and saying, “Oh this is a nice house”.

‘They asked if I had a mortgage and I said, “Yes, yes, why?”

‘They said, “Well we are really telling you this a bit early but the Queen is going to pay off your mortgage as a gift for what you have done”.

‘I thought that was wonderful. I was actually close to repossession at the time. They were going to repossess my home. So I dug myself out of that one.’

The Mirror quoted Buckingham Palace as saying the Queen’s gift would have come from her private funds.

Mr Russell’s medal will be auctioned at Dix Noonan Webb in London next month.