A senior British diplomat has said an "explosive" meeting between the US special envoy to Afghanistan and the country's president is very worrying.
US special envoy Richard Holbrooke is believed to have complained about the use of fraud in last week's election.
But Lord Ashdown, a former special envoy to Bosnia who was put forward as special envoy to Afghanistan, said foreign interference was "unhelpful".
He said foreign criticism could "de-legitimise the whole process".
Lord Ashdown told the BBC: "I think if there are doubts about corruption in the election, etc, it would be far better, at least in the first instance, to let the Afghan procedures with UN support run their course before jumping to conclusions."
Taliban 'could benefit'
Lord Ashdown added that undermining the election could have disastrous consequences.
He said: "The effect of it could be to de-legitimise the whole process.
"If it is the case that the Americans by some form or another have declared these elections illegitimate as it were, have undermined the legitimacy of the electoral process, then our capacity to be able to win back the support of the Pashtun tribes from the Taliban is lessened.
"And the people this is likely to help most are the Taliban themselves."
Mr Holbrooke is believed to have complained about the use of fraud in last week's election by some members of the president's campaign team - as well as by other candidates.
Sources report that Mr Karzai reacted very angrily to the accusations, although a spokesman for the presidential palace has denied the account of the conversation.
Mr Karzai was elected president of Afghanistan in 2004
A spokeswoman for the US embassy in Kabul denied there had been any shouting or that Mr Holbrooke had stormed out.
There have been growing doubts over the legitimacy of the Afghan elections following reports of fraud and corruption.
An investigation by the BBC found evidence that thousands of voting cards have been offered for sale and thousands of dollars have been offered in bribes to buy votes.
There have also been concerns over low voter turnout after was revealed that in one region of about 55,000 voters, only 150 people cast their ballots.
The low turnout triggered particular controversy as it follows hard on the bloodiest month so far for British soldiers in Afghanistan, who are said to be fighting in part to secure democracy for the country.
The Foreign Office has said reports about low turnout during are "anecdotal".
The Ministry of Defence insists British troops "know exactly" what they are fighting for and that their efforts had brought security to the region.
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