A UN-backed watchdog overseeing Afghanistan's elections ordered votes from 210 polling stations thrown out Monday in a move that observers said deprived President Hamid Karzai of an outright majority.
Findings from the Electoral Complaints Commission (ECC) over the August 20 vote raise the prospect that Karzai will be forced into a second round or intense diplomacy is needed to stitch together a national unity government.
The ECC said it found "clear and convincing evidence" of fraud, including entire ballot boxes with papers filled in with the same pen or same mark.
An independent US monitor said neither candidate had won outright, meaning Afghanistan could face a second round between Karzai and main rival Abdullah Abdullah, although the two are expected to come to a political settlement.
The ECC said it ordered the Independent Election Commission (IEC) -- the final arbiter of the election results -- "to invalidate a certain percentage of each candidate's votes" and results from 210 polling stations.
The ECC findings, which it said were "final and binding", could see the IEC forced to call a run-off between Karzai and former foreign minister Abdullah.
Karzai, against whom most of the ballot-stuffing allegations were made, leads preliminary results with about 55 percent of the vote, with Abdullah on around 28 percent.
The ECC refused to divulge corrected results, but one Western diplomat and a respected election monitor said Karzai's share of the vote had now fallen to about 48 percent -- not enough for outright victory.
US-based Democracy International said around one million votes cast for Karzai have been deemed invalid and adjusted figures gave the president 48.29 percent of the overall vote, with Abdullah now on 31.5 percent.
This "should necessitate a run-off election between Hamid Karzai, and the second-place candidate, Dr. Abdullah Abdullah," Democracy International said in a statement.
A total of 1.3 million votes cast were invalid, it added, based on the ECC figures, or around a quarter of the total 5.6 million votes cast.
Faced with fears that the IEC -- which Abdullah and others have accused of being pro-Karzai -- may not accept the ECC findings, there were calls for quick action to end the political deadlock.
The UN spokesman in Kabul, Aleem Siddique, said the watchdog's orders should be implemented "with haste" and called on the IEC to "move swiftly to announce either a final certified result or the requirement for a second round".
In a statement, the US embassy said: "We call on the IEC to implement these orders with all due speed and look forward to the final certified results."
And in Washington, a White House spokesman said it was "incredibly important" for the world to see a legitimate Afghan government.
"It is now up to the Afghans to demonstrate they believe in that legitimacy as well," said Robert Gibbs.
The ECC announcement comes as patience among Afghanistan's international backers wears thin, with the political crisis poised to head into a third month.
Karzai's survival -- and that of his government -- depends on the continued support of the international community, which has poured billions of dollars in aid into the country since toppling the Taliban in 2001.
Abdullah supporters have said they will not accept a Karzai victory because of the vote-rigging claims, raising fears about the next administration's legitimacy at a time of spiralling insurgent violence.
Political analyst Waheed Mujdah said even if a second round is called, the former foreign minister could be prepared to do a deal.
US President Barack Obama is considering a request from his military commanders for another 40,000 troops to boost the more than 100,000 already in Afghanistan under US and NATO command.
But White House chief of staff Rahm Emanuel and Senator John Kerry both said at the weekend that it would be "irresponsible" to send more troops to Afghanistan to fight the resurgent Taliban before the election is resolved.
Kerry, chairman of the US Senate foreign relations committee and a close Obama ally, met Karzai and Abdullah in Kabul at the weekend and returned here late Monday, according to the US embassy.