Rather, 73, had been under intense pressure in recent months over his role in a 60 Minutes report, aired at the height of the US election campaign, that questioned Mr Bush’s record in the Texas Air National Guard.

The “exclusive” turned out to be based on forged documents.

In September Rather, long accused by American conservatives of liberal bias, was forced to admit on air — after sticking by the story for several weeks — that the report that he had narrated had relied on documents that could not be authenticated. He apologised.

Rather took over one of the most coveted jobs on American television from Walter Cronkite, “the most trusted man in America”, nearly 24 years ago. He was due to step down at some time in the near future, but most media-watchers believe that the 60 Minutes debacle, quickly referred to as “Rathergate”, so tarnished his reputation that it hastened his demise. He had at least two years left on his contract.

In announcing his retirement from the news anchor chair next March, Rather made no mention of the scandal. He said that he would continue to work for CBS as a reporter. “I have always been and remain a hard-news investigative reporter at heart,” he said. “I now look forward to pouring my heart into that kind of reporting full time.”

The discredited report cited documents purported to be from Lieutenant-Colonel Jerry Killian, one of Mr Bush’s commanders in the Texas Air National Guard. The documents claimed that Mr Bush had refused a direct order to take a medical exam and that Mr Killian felt pressured by superiors to “sugarcoat” an evaluation of Mr Bush, then a First Lieutenant.

Within hours of the report, however, several experts said that the memos were probably fakes and appeared to have been generated by a modern word processor, using a print font that did not exist on Vietnam-era typewriters.

Then it emerged that the CBS source was Bill Burkett, a retired Texas Air National Guard member, who had a history of making allegations about Mr Bush’s record in the Guard. He compounded CBS’s problems when he said later that he had agreed to turn over the documents to the network if CBS would help to arrange a conversation with John Kerry’s campaign staff.

A report by a two-man independent investigative panel into the affair is due imminently and is expected to be critical of Rather, who has worked at CBS News for more than 40 years. He made his name as a reporter covering the Nixon White House.

His near-quarter-century at the helm of CBS Evening News is the longest by a presenter of any network for an evening broadcast. Rather dominated the ratings after taking over from Cronkite during the 1980s, but he was eclipsed first by Peter Jennings, of ABC, and then by Tom Brokaw, of NBC. These days his evening news broadcast generally runs a distant third in the ratings.

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