Prof Stephen Hawking has said he became so ill while writing A Brief History Of Time that doctors offered to turn off his life support machine, it has been reported.

In a new film, the renowned physicist tells of how in 1985 he thought that contracting pneumonia would stop him from completing the book which went on to sell 10m copies, the Sunday Times reported.

But his first wife, Jane Hawking, refused to end her husband's life and demanded doctors in Switzerland return him to Cambridge.

Although the life-saving treatment left him unable to speak, the motor neurone disease sufferer went on to publish the book which brought him worldwide fame, the newspaper added.

"The doctors thought I was so far gone that they offered Jane [the option] to turn off the machine," Hawking, 71, says in the documentary. "The weeks of intensive care that followed were the darkest of my life.

"But slowly the drugs worked, though a small incision in my throat robbed me of my ability to talk. I was then put on a ventilator and hopes of finishing my book seemed over."

Hawking speaks openly about his two failed marriages, his second to the nurse Elaine Mason, and his constantly precarious health in the documentary which coincides with the release of his biography, the newspaper reports.

He reveals that he has rekindled a friendship with his first wife, who also appears in the documentary and shares her heartbreak at the breakdown of their marriage.

The former Lucasian professor of mathematics at Cambridge University hopes to travel to space with Sir Richard Branson's space tourism business, Virgin Galactic.

The documentary film, Hawking, is due to be released later this year.