A US-sponsored survey of Afghan voters indicated yesterday that Hamid Karzai, the incumbent, won Saturday's presidential election by a wide margin.

The figures were released as the most prominent candidate running against Mr Karzai dropped out of a boycott called by opposition candidates to protest against polling irregularities.

The poll of about 15,000 voters by the International Republican Institute, a non-profit organisation that promotes democracy, may be the only indication of the election's result for many days, since ballot-counting may not begin until October 16.

It will be a boost to Hamid Karzai and his US allies, who were anxious that the moderate president win with a strong mandate.

With 10,050 surveys counted, the organisers said Mr Karzai had a lead of 43 percentage points over Yunus Qanooni, a former cabinet minister, in second place. They did not say what percentage of votes either candidate had won.

The survey was conducted on behalf of the IRI by Williams and Associates, a Massachusetts-based polling group, and was paid for by USAid, the US government's development arm. The IRI is not affiliated with, but has strong ties to, the Republican party. The survey involved interviewing a random sample of voters in 177 polling centres around the country.

Full findings are due to be issued tomorrow, and organisers expected the survey to reflect the result of the Afghan vote within one or two percentage points.

After a meeting yesterday with Zalmay Khalilzad, the US ambassador to Kabul, and Jean Arnault, head of the UN in Afghanistan, Mr Qanooni said he had agreed to accept an independent investigation into irregularities at Saturday's poll for the sake of "national unity and reconciliation".

His decision spells the collapse of a two-day boycott by the 15 candidates, who branded the process illegitimate after running against Mr Karzai.

Kent Patton, a senior adviser to the IRI in Afghanistan, said he hoped the survey would provide "a measure of confidence in how the process proceeded and eventually in the result".

The survey indicated that the candidate in third place had 5 per cent of the vote and that most of the other 13 candidates had less than 1 per cent. Most respondents said the situation in the country was improving, and nearly 50 per cent said their top priority was disarming militia commanders.