British acting veteran Sir Patrick Stewart feared he would collapse and die as he took part in the torch-carrying celebrations ahead of the 2012 London Olympics, revealing he had no idea how physically demanding the role would be.

The Star Trek icon, who used to be an athlete at school, put on his running shoes to do a little light training for the big event, but he was given a shock on the big day when he realised he would be jogging up a hill, days after his 72nd birthday.

His fears for his health grew after learning how heavy the beacon actually was, but he forced himself to fake a smile as he embarked on the quarter-mile (402.34 metres) run in Croydon, south London, where he was cheered on by crowds of locals.

He tells U.S. talk show host David Letterman, "I was asked to do this; it was the first day the torch came to London. I'd had about six weeks' warning, so I went into training. I had a half hour of flat, quiet country road near my house (in England) and I would go up and down it. I thought, 'If I can do half a mile...', and I made myself a kind of imitation torch. It wasn't heavy enough and I would pound up and down the country lane.

"We got to the morning... (and) they (organisers) dropped me off at the bottom of the hill... It was horrible because I hadn't trained for a hill! I was so upset and angry and thought I might die, but I was determined (that) if I died, I would throw that damn torch at somebody before I went down! (It was) very heavy."

However, despite his struggles, Stewart insists it was an honour to be involved in the momentous occasion.

He says, "As a one-time athlete and a fanatic about the Olympics, to be carrying the Olympic torch, if only for a quarter of a mile, was great."

Other British stars who took part in the torch run around the U.K. included actor James MCAvoy and former Spice Girls singer Melanie Chisholm.