The move by CBS came after an independent investigation into the 60 Minutes report, shown at the height of the US election campaign, that questioned Mr Bush’s record in the Texas Air National Guard.

That report, based on documents that CBS could not then authenticate, was the result of “myopic zeal” and extraordinary sloppiness which led the network to disregard the basic principles of journalism, the investigation concluded. That failure was compounded by a “rigid and blind” defence of the report, the investigators, Dick Thornburgh, the former US Attorney-General, and Louis Boccardi, the former Associated Press president, added.

The 224-page report did not directly fault Dan Rather, the veteran CBS anchorman who narrated the story. But media analysts believe that the journalistic debacle, which happened in September, hastened the end of Mr Rather’s career.

Despite nearly two years left on his contract, Mr Rather, long accused by American conservatives of liberal bias, announced in November that he would step down from the anchor’s chair in March. The affair is the latest blow to CBS’s once venerable reputation.

The discredited report cited documents purporting 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 Lieutenant-Colonel Killian felt pressured by superiors to “sugarcoat” an evaluation of Mr Bush, then a first lieutenant. But several experts said the memos were probably fakes and appeared to have been generated by a modern word processor, using a print fount that did not exist on Vietnam-era typewriters.

It then emerged that the CBS source was Bill Burkett, a retired Texas Air National Guard member, who had a history of making claims about Mr Bush’s record in the Guard.

The network dismissed Mary Mapes, producer of the report; Josh Howard, executive producer of 60 Minutes Wednesday; Mary Murphy, his deputy; and Betsy West, a senior vice president.

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