Christopher Reeve, the ’Superman’ film star who continued to believe he would walk again, has died nine years after being paralysed in a riding accident. Reeve, 52, became the international spokesman for campaigners calling for stem cell research to those with spinal injuries.

The former actor appeared so indomitable in his struggle to stay alive after his paralysis that his death came as a complete shock to his friends yesterday.

Margot Kidder, who played Lois Lane opposite him in the 1978 film Superman, said she had imagined he was "doing so well".

"I am just heartbroken and so sad," she said. "I just don’t know what to say right now. He grew into such a glorious example of triumph over everything."

His mother, Barbara Johnson, indicated that her son’s perpetual brave face was becoming increasingly hard for him to bear. She said: "He put up with a lot. I’m glad he is free of all those tubes."

Senator John Kerry, the presidential candidate, said the country had lost a man who was "truly America’s hero".

Reeve fell into a coma after going into cardiac arrest at his New York home where, in a wheelchair and stimulated by what he described as "massive doses of calcium for my bones", he worked assiduously, with a word-activated computer, spreading the word on the stem-cell research that might have saved him.

But for those close to him it was becoming clear that new threats were undermining his health.

He told the BBC World Service last month of three "brushes with death", including bacteria appearing in his blood stream "virtually destroying the immune system, destroying the red cells and wreaking havoc in the body". Antibiotics saved him.

He said: "I am now nine and a half years post-injury, but due to ageing as well as a number of other factors in the last year, I now face a number of health issues which have not plagued me before."

Reeve had abandoned his treadmill, swimming pool therapy and rides on a computerised bicycle. He said: "I am facing some very enormous challenges."

Yet his courage shone through as he added: "There is not a boring moment in my life."

In a documentary to be shown this month, he confessed: "My recovery appears to have plateaued."

His wife Dana, speaking to the British film makers TWI, recalled a gruelling time last winter, with sores and "skin breakdown" making it hard on his "quality of life".

She said: "He had to spend really limited time up, going to bed at 6pm. We would try to keep it cheerful and have dinner in the bedroom and things like that, but it was very, very tough."

Michael Winner, the film director, called Reeve the "archetypal movie star". He said: "I think he grew to personify a heroic struggle against disability. We all kind of believed that we would one day see him walk again. Instead, we saw him die really very young."

In making audiences believe man could fly, Reeve spent 18 months in England shooting Superman. Susannah York, who played his mother in the film, recalled Reeve’s dedication to his comic-book character.

"Chris and I used to have lunch, and talk, and I grew to admire him enormously. I just saw somebody so dedicated to what he was doing. He had such a love of the Superman comics, and a sense of duty and responsibility to make our adventures as real and as marvellous as they had been for the readers.

"He spent an enormous amount of time keeping fit, and being right on his part. He was a very focused man, and earnest, which wonderfully suited the Clark Kent side of his character. He also had a real twinkle about him.

"He was very generous-spirited - and you also felt that he had a strong moral purpose, which one saw in his later life. I think that he truly believed that he would walk again - and, who knows, he might have, if it had not been for the enormous strain it put on his heart."

When approached to play Superman, Reeve initially thought the role was beneath him. "I was sort of a snob about it. I thought it would be kind of hokey, and I didn’t quite get it that this guy was a cultural icon."

But the 6ft 4in actor seized the opportunity, lifting weights to add 30lbs of muscle for the screen test, in which he outdid 200 other aspirants.

While filming in London, he met Gae Exton, a modelling agency executive, with whom he had two children, Matthew, now 25, and Alexandra, 21. The couple parted without marrying. Then, while working in summer repertory, he met Dana Morosini while she was performing in a cabaret. They married and had a son, Will, now 11.

With the last two Superman sequels less successful than the others, his career was in flux when, on May 27, 1995, in a competition in Virginia, his horse suddenly stopped before a jump, and he flew from the saddle.

If a bystander had not swiftly opened his air passage, he would have died. His head later had to be reattached to his spinal column.

He said: "I remember going out to the starting box, and then I’m a complete blank until three days later."

Robin Williams, a lifelong friend, came to his hospital room in a blue scrub hat and yellow gown. He said in a Russian accent while snapping his rubber gloves: "I’m a proctologist, here to examine you."

Reeve, who said that he had contemplated "pulling the plug", recalled: "For the first time after the accident, I laughed. And with that, I knew I was going to be OK."