The son of Stephen Hawking has admitted putting swear words into his father’s speech machine as a prank growing up.



Tim Hawking, 36, also said he would use his father’s wheelchair as a go-kart as a child.



He was speaking to comedian Dara O’Briain for a BBC documentary on his father to be broadcast on Tuesday night.



Hawking’s speech deteriorated as a result of his motor neurone disease. Tim, born in 1979, said in the interview that it was only after his father received his speech box (he lost the use of his voice completely following a tracheotomy after contracting pneumonia) that he felt able to communicate with him.



Tim, who works in the marketing department for toy company Lego, said: “My dad was able to speak with his own, natural voice for those first years, but it was incredibly difficult to understand what he was saying – particularly for me at such a young age.



“As a three-year-old, I had no understanding of what he was saying. I didn’t really have any communication with him for the first five years of my life.



“It was only when he got his voice synthesiser that I was actually able to start having conversations with him.



“It was somewhat ironic that Dad losing his voice was actually the start of us being able to form a relationship.”



He added that the pair would endure arduous battles in board games.



“My father is hugely competitive and he certainly wasn’t the easiest opponent at any game, particularly chess,” he said.



Tim is the third of Hawking’s children with his first wife, Jane Wilde. His sister Lucy, 45, an author, was also interviewed for the documentary.



The programme, Dara O’Briain Meets Stephen Hawking, will be shown on BBC1 on Tuesday at 10.35pm.



















































View photos Tim Hawking standing alongside sister Lucy Hawking More