As the Kennedy assassination's 50th anniversary approaches, an esteemed academic has called into doubt the basis for a range of conspiracy theories surrounding it.

President John F. Kennedy was shot dead on 22 November 1963, as he and his wife rode in a motorcade in Dallas, Texas.

Many of the theories stem from the idea that a fourth shot was fired, in addition to the three by assassin Lee Harvey Oswald. This was backed by the 1979 House Select Committee on Assassinations, and led to swirling questions about who fired that shot.

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It was based on a recording from a police radio, which was supposedly near the crime scene.

But Larry Sabato, Professor of Politics and director of the University of Virginia's Centre for Politics, has cast doubt on that theory.

He told The Times the evidence used by that committee was “completely flawed”. He said modern technology has revealed the fourth sound was not gunfire, but the rattling of the microphone. He also questioned whether the radio which provided the recording was even in the motorcade.

Dr Sobato, who has also taught at Oxford and Cambridge, said: “It is impossible to record gunshots from that distance on equipment this primitive.

“I think some people on the committee wanted to find a conspiracy. If you find a fourth shot you have found a conspiracy.”

His new book, The Kennedy Half-Century, also claims that 75 per cent of Americans still reject the idea that Oswald acted alone.