But speaking to Al Jazeera on Thursday, Galbraith said the story was "a phoney which was being put out for the purposes of obscuring the real issue - the mishandling of the elections by the United Nations".
The UN, Galbraith said, had failed "to take steps that might have prevented the fraud in the Afghan elections" and then afterwards decided to try to cover it up.
|Galbraith was sacked from his post with the UN in September [EPA]
He said his dismissal had become a source of "acute embarrassment to the United Nations when it turned that what I was saying was 100 per cent correct."
Galbraith left Afghanistan in early September after the first round of the country's presidential elections and was fired from his post weeks later.
He has accused Kai Eide, the Norwegian diplomat heading the UN mission in Afghanistan, of trying to conceal the degree of fraud in favour of Karzai's re-election.
In October, after Galbraith was sacked, a UN audit stripped Karzai of almost a third of the votes he had taken in the first round, forcing him into a second-round run-off.
However, days later Karzai was re-appointed to office uncontested after his rival, Abdullah Abdullah, withdrew from the run-off, citing widespread corruption.
Speaking from Bergen in Norway, Galbraith said the UN's handling of the elections had become a "huge issue" in determining the future direction of Afghanistan.
"These fraudulent elections have given the Taliban their greatest victory in eight years," he said.
The vote, he said, had resulted in "a prolonged and unnecessary political crisis, and it need not have occurred had the UN done its job and insisted on honest elections."
According to the New York Times, Eide, who is set to leave his job as head of the UN mission in Afghanistan by early 2010, said Galbraith's departure came immediately after he rejected a proposal to replace Karzai and install a more Western-friendly figure.
Eide was quoted as saying he had told Galbraith the plan was "unconstitutional, it represented interference of the worst sort, and if pursued it would provoke not only a strong international reaction" but also civil insurrection.