Crowned light-heavyweight champion of the world in 1948, Mills was a darling of the media and a hero to millions. Wealthy, good-looking and overflowing with charm, his success continued long after he hung up his boxing gloves.

For years, he was in massive demand both on radio and television. He wrote a boxing column for a Sunday newspaper, picked up acting work, including roles in two Carry On films, and in partnership with former actor Andy Ho opened one of London's first Chinese restaurants.

But in July 1965 he was found dead in the back seat of his car, just yards from the restaurant, which by then had been converted into a nightclub. He had been shot once in the head; a rifle rested between his knees.

Police claims of a suicide were dismissed as a cover-up by friends and family. His widow, Chrissie, was convinced he had been killed as an example by gangsters who were extorting a fortune from club owners at the time.

Two weeks before his death, she claimed, a former professional wrestler had had his West End venue smashed to pieces after refusing to pay protection, while another club owner had been doused with petrol and threatened with matches.

Another theory was that Mills had been killed by Chinese Triad gangsters, who wanted to convert his club back into a restaurant.

It later emerged that, in the days before his death, Mills had asked several prominent figures in London's gangland whether they could provide him with a pistol, seemingly aware that his life was in danger. Unable to secure a proper gun, he went to see a friend, Mary Ronaldson, who ran a rifle range at Battersea fun fair. He asked if he could borrow a weapon, telling her he had been invited to a charity fête and wanted to dress up as a cowboy. It was that gun that was found in his car.

The murder theory was supported by the fact that Mills had been shot directly in the right eye, a highly unusual site in the case of suicide. The fact that the eye was open when the bullet struck only added to the mystery. Professor David Wingate,who was resident medical officer at Middlesex Hospital the night Mills's body was brought in, carried out an examination. He remains convinced that someone had taken the gun off Mills and shot him with it, but he was not called to give evidence at the inquest.

However, respected author James Morton has probed into Mills's background and now believes that the boxer took his own life through a combination of his own depression and fears that he was about to be killed by the Kray twins.

'The Krays didn't actually kill him, but the threat that they were about to move against him was enough to push him over the edge,' says Morton. 'He was having problems at home with his relationship with his wife and he was suffering from the most dreadful and persistent head aches as a result of too many punches to the head.

'There is some evidence that he was also developing speech difficulties, which had put an end to his broadcasting career. He had been suffering with dizzy spells and bouts of depression for some time, but by then it had got much worse.'

The comedian Bob Monkhouse spoke to Morton a few months before his death last year. Monkhouse, whose regular writing partner, Denis Goodwin, was a manic-depressive and ultimately committed suicide, was convinced that Mills, whom he knew well, was suffering from the same condition.

'Freddie had a very dark side. Denis felt a tremendous kinship with him and they palled around together. Denis had this unabashed glee and terrible moments of despondent mood, exactly the same as Freddie,' said Monkhouse, speaking for the first time about the boxer.

'Denis said his depressions came after a successful show and Freddie that his came after a successful fight, because he felt he could never do any better.'

According to Morton, the final part of the puzzle lies with former Italian hardman Robert 'Battles' Rozzi, now in his eighties. Rozzi, who had a long involvement in the underworld, got to know the Krays during a spell in prison.

'In July 1965 Ronnie Kray came to me and said that Mills's partner, Andy Ho, wanted him [Mills] out of the club and that there was money in it for them if they got him out. I said Freddie wasn't going to take any nonsense from them and he'd have hit them. Then they'd have had to up the ante, so to speak, and it would have got out of hand.

'I knew Freddie and said I would go round and have a chat with him. I went to see him a couple of days later. I said: "Your back's to the wall. Give a little or there'll be trouble." I left him in an uncertain state of mind. He didn't say "Go and fuck yourself", but he didn't say he'd agree.'

Rozzi told Kray to wait a few days to see if Mills took the hint. He told him: 'If need be, do what you have to do, but I didn't want any part of it. Five days later, he's dead in the car. Ronnie said it was nothing to do with them, and I believed him.'

According to Morton, Mills simply felt he had no way out and the evidence that suggests his death could not have been suicide is flawed. 'He had reached the end of his tether. He was no longer the popular figure he once was and he was in desperate financial trouble because the club was a black hole swallowing money.

'Mills suspected Ho was stealing from him and may have confronted him. Ho turned to the Twins for help and, when Mills learnt they were going to back Ho rather than him, it was the last straw.'

· 'Fighters' by James Morton is published by Time Warner at £17.99