CBS has made an embarrassing admission. The network says it cannot be sure that documents used to support a 60 Minutes story on U.S. President George Bush and his military service were authentic.

CBS News and its anchor, Dan Rather, said mistakes had been made. He spoke on the evening newscast and apologized for airing a story that could not be substantiated.

"It was an error that was made, however, in good faith and in the spirit of trying to carry on a CBS News tradition of investigative reporting without fear or favoritism," Rather said.

The story about Bush and his Vietnam War-era National Guard Service aired on Sept. 8. Almost immediately after the story aired, experts raised questions about the authenticity of the memos used in the feature.

The memos were reportedly written by Bush's late squadron leader. However, experts said they appeared to have been written on a computer, not a typewriter, which would have been used during the 1970s. The military leader's former secretary also said she believed the memos were fake.

CBS News said in a statement that former Texas Guard official Bill Burkett, mentioned as a source for the documents, "has acknowledged that he provided the now-disputed documents."

Burkett, speaking to Rather in an interview aired on Monday's newscast, denied forging the documents. But he did admit to misleading CBS staff about the source of the memos.

"Your staff pressured me to reveal that source," Burkett said. "And I simply threw out a name that was basically to get a little pressure off for the moment."

Rather says no one has disputed the story's premise: That the future president had pulled strings to get an easy National Guard assignment and failed to satisfy the requirements of his service.

Bush's National Guard service has been the source of much speculation. The memos used in the original 60 Minutes report were said to be written by Lt.-Col. Jerry Killian, suggesting he was to "sugar coat" the performance ratings of the young Bush.

The memos also suggest Bush ignored direct orders to take a physical. Killian died in 1984.

The report also featured former Texas Lt. Gov. Ben Barnes. He claimed that, at the urging of a friend of the Bush family, he pulled strings to get young Bush into the Guard.

Bush has always maintained that he didn't get any special treatment. He was honourably discharged.

The admission by CBS comes just six weeks before the U.S. presidential election, and could help to boost the president's support.

The White House said Monday that the affair raises questions about the connections between CBS's source and Democrat John Kerry's presidential campaign.

White House press secretary Scott McClellan said Bush was told about the CBS statement as he flew to Derry, N.H.

"There are a number of serious questions that remain unanswered and they need to be answered," McClellan said.

"Bill Burkett is a source who has been discredited and so this raises a lot of questions. There were media reports about Mr. Burkett having senior level contacts with the Kerry campaign."

Bush and rival Kerry have sparred over each other's military records in the past. Kerry is a decorated war veteran, but some critics have questioned the circumstances leading to the incident that saw his awarded with a Silver Star for battlefield heroism, as well as his three Purple Heart medals.

With files from The Associated Press