By Alan Baldwin

LONDON (Reuters) - Formula One great Stirling Moss announced his retirement from motor racing at the age of 81 on Thursday.

The Briton, widely regarded as the greatest driver never to have won the Formula One championship, made his decision in the Le Mans pitlane where he had been due to race his own restored 1961 Porsche RS61 in a Legends race.

"This afternoon I scared myself and I have always said that if I felt I was not up to it or that I was getting in the way of fellow competitors, then I would retire," he declared on his website (www.stirlingmoss.com).

"I love racing, but now it is time to stop."

Four times a Formula One world championship runner-up, Moss ended his professional career after a bad accident at Goodwood in 1962 but had continued racing historic cars for his own pleasure.

The Porsche he would have raced at Le Mans was damaged at Laguna Seca in California last August when he spun off and was hit by a Lotus.

Moss turned professional at the age of 18 in 1948, racing a Cooper 500 two years before the Formula One world championship started, with his 1955 Mille Miglia victory for Mercedes a career highlight.

He survived a three-storey plunge down a lift shaft at home in March 2010, breaking both ankles and four bones in his feet, but recovered to get back behind the wheel again by July.

Moss was a contemporary of the late Argentine world champion Juan Manuel Fangio and won 16 grands prix, one more than McLaren's Lewis Hamilton and Red Bull's reigning world champion Sebastian Vettel have to their credit.

A keen follower of social media and the latest technology, Moss had earlier advised more than 13,700 followers on Twitter of his decision.

"It's official the white Patey helmet has been hung up for good. Stirling Moss has retired from competitive racing," the message declared.

(Reporting by Alan Baldwin, editing by Ed Osmond)