President Karzai could be forced to form a coalition government in Afghanistan or face a second round of voting after electoral fraud officials slashed his share of the vote to below 50 per cent.

After more than eight weeks investigating 2,584 allegations of election fraud, misconduct and vote-rigging, the UN-backed Electoral Complaints Commission (ECC) is expected to cut Mr Karzai’s lead to about 47 per cent today. If the figure is upheld as final, it would mean that the President would have a run-off with his closest rival, Dr Abdullah Abdullah.

Mr Karzai insisted that reports of irregularities were fabricated and politically motivated. Diplomats fear that he will reject the commission’s rulings. A British official said: “There are strong indications that whatever the ECC decides, it won’t be accepted because of pressure being applied by the Karzai campaign team.”

Provisional results from Afghanistan’s Independent Election Commission, which was implicated in the fraud, gave Mr Karzai 54.6 per cent, ahead of his former foreign minister on 27.8 per cent. European Union monitors said that up to 1.5 million votes were cast illegally.

Worsening security and the onset of winter would make a second round difficult and there is no guarantee that it would be fairer than the first. It would also prolong an awkward political vacuum in Kabul while President Obama debates whether to send 40,000 more US troops to the country.

Western officials are putting pressure on Mr Karzai to form a coalition government with Dr Abdullah. “The maths is incontrovertible,” a British official said. “Karzai has got below 50 per cent, necessitating a second round. The only way he could win outright is through political interference.”

A second official involved in the recount confirmed that Mr Karzai’s tally had fallen but warned that it could rise in the final phase of the investigation. The ECC has finished an audit of 3,377 suspect polling stations and investigated 893 “Category A” complaints — those deemed serious enough to alter the result significantly. They are working out what to do with about 2 per cent of the suspect votes, which were kept out of the published results.

A commissioner who supported Mr Karzai resigned from the complaints commission last week, citing foreign interference when it became clear that a second round would be likely.

Mr Karzai accused Richard Holbrooke, the US envoy to Afghanistan and Pakistan, of trying to force a second round against the interests of his country in a meeting after the August 20 poll.

If the President rejects the rulings, diplomats fear that Dr Abdullah’s supporters will riot. They have threatened to hold protests “with Kalashnikovs” if Mr Karzai gets a first-round victory, insisting that he could have done it only by cheating.

Both men had maintained that they would never negotiate. One ally of Dr Abdullah said that any deal with the Karzai camp would be “like drinking dirty water”. Their rhetoric has softened slightly with the arrival of a former US Ambassador to the UN, Zalmay Khalilzad — of Afghan origin — who is charged with organising talks between them.

Dr Abdullah said that he would be open to discussions once the final results were announced. He is understood to have demanded control of at least ten government ministries as his price for joining the Government.

“The reality is that a second round would be extremely difficult to conduct because of the weather and insecurity,” the British official said. “The only way forward is for the leading candidates to come to some sort of arrangement.”

There is little support for a second round among Afghans and although some analysts believe that it is the only way to bring credibility, others insist that it would be worse than the first, with apathy, insecurity and snow keeping people away.

“Why would it be any different?” said an American election observer who took a dim view of Karzai, Abdullah and the ECC. “I think come Sunday we’re going to get an announcement of a second ballot. But before that happens we’ll get a deal between the two candidates that will satisfy one of them enough to pull out.”

Four low-ranking election officials were dismissed for their involvement in the fraud but the UN said that more than half of almost 380 district election chiefs were being replaced in preparations for a second round.

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