A former senior United Nations diplomat in Kabul has launched a scathing attack on the UN's handling of Afghanistan's disputed elections, claiming that almost one in three of the votes cast for president Hamid Karzai were fraudulent.

Writing in today's Washington Post, Peter Galbraith, the former deputy head of the UN mission in Afghanistan, singled out his former boss Kai Eide for criticism, saying that he had deliberately downplayed the level of cheating in an election where in one region "10 times as many votes were recorded as voters actually cast".

Galbraith, who was sacked last week after his disagreements with Eide about how to deal with electoral fraud became public, said the extraordinary level of fraud in the August vote "has handed the Taliban its greatest strategic victory in eight years of fighting the United States and its Afghan partners".

The election was a "foreseeable train wreck", he said, with Eide – the Norwegian diplomat in charge of the UN mission – standing idle as Afghan election authorities and ministers loyal to the president avoided taking steps that could have reduced massive fraud.

The extraordinary intervention could torpedo what many diplomats in Kabul suspect is an attempt by Eide and the US to minimise further controversy over fraud allegations and move quickly to declare Karzai the re-elected president of Afghanistan.

Opposition politicians, including Abdullah Abdullah, the second-placed candidate, who wants to see a run-off vote, have seized on remarks made by Galbraith since he was sacked by Ban Ki-moon, the UN secretary general, last Wednesday.

Yesterday Abdullah accused Eide of "giving a green card for fraud to determine the outcome of the election".

A war of words between Galbraith and the UN – which has attempted to characterise the row as a "personality dispute" – has been gradually escalating since he was sacked last week.

In a letter to Ban, which was leaked to the New York Times, Galbraith made a number of devastating allegations against Eide, including the claim that the Norwegian diplomat ordered him not to hand over to election officials information that showed turnout had been tiny in the south, where the Taliban intimidation campaign against voters was most effective.

He also said Eide told him to stop lobbying for the elimination of "ghost polling stations" – voting centres in areas of the country that were too dangerous to actually open, but which nonetheless received ballot papers that could be filled out by corrupt officials.

Galbraith also claimed Eide prevented him from trying to stop the Independent Election Commission from abandoning its own safeguards, which would have excluded fraudulent ballots from the count, probably reducing Karzai's score to below 50%, forcing a second-round vote.

Eide told him to back off, Galbraith said, after Karzai ordered his foreign minister to protest that the American was interfering in Afghan affairs. He said the Afghan government threatened him with expulsion.

His article in the Washington Post went even further with its claim that a third of Karzai's votes were fraudulent. If true, that would mean the president received well under the 50% of all votes required for him to win on the first round.

Preliminary results give Karzai 55% and Abdullah 28%.

The Election Complaints Commission has ordered an audit of 10% of the 3,063 votes that have been deemed to be suspicious because of a very high turnout or where nearly all the votes went to a single candidate.

Galbraith also warned of the huge political dangers if the outcome of the vote is not accepted by the Tajiks, Afghanistan's second-largest ethnic group, who are predominant in the north and seen as Abdullah's main constituency.

"If the Tajiks believe that fraud denied their candidate the chance to compete in a second round, they may respond by simply not recognising the authority of the central government," he said.

He also said the high level of fraud "virtually guarantees that a government emerging from the tainted vote will not be credible with many Afghans", destroying President Barack Obama's hopes of having a legitimate partner in the country to help implement his strategy.