The son of esteemed theoretical physicist Stephen Hawking has admitted to tampering with his father's speech device as a child - and even adding curse words to its vocabulary.

Tim Hawking, 36, also said he would use his father's wheelchair as a go-kart. He was speaking to comedian Dara O'Briain as part of a BBC documentary on Stephen Hawking which was aired on Tuesday.

Tim revealed that in the years before his father received his voice synthesiser, he could barely understand him:

“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.”

Tim is the third of Hawking’s children with his first wife, Jane Wilde.