The US TV network CBS has said it now has doubts over the authenticity of documents it used in a report about President Bush's military service.
Doubts about Mr Bush's service were first raised in the 2000 election
The presenter, Dan Rather, said he apologised for a mistake in judgement.
The 60 Minutes programme questioned Mr Bush's record during the Vietnam War.
The White House said the affair raised questions about connections between CBS' source and Democrat John Kerry's presidential campaign.
The service records of both Mr Bush and Mr Kerry, a Vietnam veteran, have become a major issue in the US presidential campaign.
Correspondents say the revelation has been a major blow to the credibility of CBS and Mr Rather in particular.
After the programme was aired, experts suggested the documents CBS used to suggest Mr Bush falsified his service record could only have been created on a word processor not available at the time they were purportedly written.
The network has not said that the documents were forgeries, but Mr Rather said he no longer had confidence in them.
"We made a mistake in judgement, and for that I am sorry," he said.
CBS News President Andrew Hayward said in a statement that the apology came after a source for the programme - former Texas National Guard official Bill Burkett - admitted misleading the producer by giving her a false account of the documents' origins.
"Based on what we now know, CBS news cannot prove that the documents are authentic, which is the only acceptable journalistic standard to justify using them in the report," he said.
The documents said Mr Bush was suspended from flying for the Texas Air National Guard during the Vietnam war because he failed to meet its standards.
Mr Bush trained as a pilot while a member of the Texas and Alabama air national guard but never flew in combat.
Documents already released by the White House show that Mr Bush was suspended from flight status in 1972 for not having a medical test - but do not mention his alleged failure to comply with national guard standards.