He thrust chess on to the front pages when he beat the defending Soviet world champion at the height of the Cold War. But Bobby Fischer, the prodigy from Brooklyn who died this month, was a deeply disturbed individual with a cruel streak that he turned on his own mother.

Unpublished letters seen by The Timesnow throw new light on the origins of Fischer’s precarious mental state and his obsession with beating Boris Spassky at the so-called match of the century in Reykjavik in 1972. He did not know who his real father was, and he deeply resented his mother for being an active communist under constant surveillance by the FBI.

Regina Fischer, rejected by her son and filled with anguish at their separation, continued to follow his meteoric career, turning up in Iceland disguised in a blonde wig to witness his finest hour, in defiance of his orders to stay away.

For three years between 1957 and 1960 Regina wrote regularly to Joan Rodker, now a retired journalist, photographer and film-maker, disclosing some of the hurt felt by a mother disowned by an extraordinary son. The two women had met as idealistic communists living in Moscow in the 1930s and remained friends.

“I have found I am not very necessary or useful to Bobby, and actually my presence is an irritant to him,” Regina wrote in May 1959. “Merely being there, or in the vicinity, is enough.”

When Fischer became US champion at the age of only 14, Regina wrote to Mrs Rodker describing him as “temperamental, unable to get along with others, without friends his age, and without any interests other than chess”. Yet she often wrote in more glowing terms, expressing her maternal pride when her boy won yet another tournament.

By the time Fischer was 16, his mother decided to pursue her own obsession of training in medicine. She said that her son would have to live in their Brooklyn apartment without her: “It sounds terrible to leave a 16-year-old to his own devices, but he is probably happier that way. Maybe he is better off without my nagging him to go out for sports, etc, eat, get through his homework, go to bed before 1am, etc. I am tired of being a scapegoat and doormat.”

Good riddance, Fischer apparently thought. In an interview withHarper’s magazine in 1962 he accused his mother of being a “square”, adding: “I don’t like people in my hair, so I had to get rid of her.”

Despite the rift Regina continued to fight her son’s corner, travelling to Washington soon after their parting to stage a lone five-hour protest outside the White House urging President Eisenhower to allow a American chess squad to visit East Germany. Her voice was heard, and the players set off for Leipzig, led by her son.

Mrs Rodker, now aged 92, believes that Fischer’s resentment was born of Regina’s obsession with left-wing activism, leading to a perceived failure to be there for him as a mother. “She was an ardent communist. The Soviet Union could do no wrong, which was why Bobby was so ardently against it,” Mrs Rodker told The Times.

“She cared for him but was very busy with her own life, and what did Bobby get? I think that in a way the chess supported him, but it also became an obsession. I think his hatred came because he felt that she was not a mother. She was very restless and always creating trouble; in their block of flats she was always protesting. She must have thought that the whole world was against her.” Regina lived in London in the 1970s. She was arrested during a protest outside the Home Office and went on hunger strike.

Fischer grew up without a father figure. Paternity was officially held by Regina’s first husband, Hans-Gerhardt Fischer, a German who had fought on the republican side in the Spanish Civil War and who lived for a time with his new bride Regina, Swiss-born but of Polish-Jewish descent. They divorced in 1945.

Among 750 pages of files released by the FBI on Regina’s death from cancer in 1997 is a suggestion that Regina and her husband had been separated since 1939, four years before Bobby’s birth, and that the boy’s real father was Dr Paul Nemenyi, a Hungarian physicist, suspected by the US authorities of being a communist, despite working on the US atom bomb project during the Second World War.

Mrs Rodker recalls meeting Regina, Dr Nemenvi and the infant Bobby during a visit to the US. “She did not say this was the father, but he was obviously fond of Bobby and he used to hold him like a real father.”

There were rumours of a rapprochement between Bobby and his mother in later years but he continued to live a strange and hermetic existence, occasionally surfacing to deliver antisemitic rants, a final sad rejection of his own heritage.

David Edmonds, co-author of the respected biography Bobby Fischer Goes to War, said that the letters were of great interest in helping to solve unanswered questions about Bobby’s life; they provided further anecdotal evidence that he was really Bobby Nemenyi.

Mr Edmonds said: “Regina did not have a normal child and she did not know what to do with him. From the age of 6 he was effectively lost to the game of chess. Nemenyi might well have blamed her for that. She must have been at her wits’ end.”

If Fischer had been a child today, Mr Edmonds suggested, he would probably have had a diagnosis of Asperger’s syndrome.

But then Reykjavik might have been very different, the impregnable fortress of Soviet chess might never have been breached, and America might never have enjoyed such a huge propaganda victory, delivered into her hands by an odd son.

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