Hamid Karzai was officially declared the winner of Afghanistan's first-ever presidential election Wednesday after a three-week probe into vote fraud found no grounds to invalidate his triumph.

The joint U.N.-Afghan electoral board confirmed that the American-backed incumbent had clinched a five-year term as the country's first popularly chosen leader.

"His excellency Hamid Karzai is the winner of the election," board chairman Zakim Shah said at a ceremony in the capital. "We are announcing the first elected president of Afghanistan."

Karzai received 55.4 percent support in the Oct. 9 election, 39 percentage points ahead of his closest challenger and enough to avoid a runoff.

A spokesman for Karzai, who was in the United Arab Emirates (search) for the funeral of its late president, said he was "very glad to finally have the result we wanted" and appealed to rivals to put a bruising campaign behind them.

"We are starting a new life, a new Afghanistan and we hope everyone with cooperate with its reconstruction," Elmi said.

Karzai was expected to make a victory speech in the Afghan capital Thursday.

However, his nearest rival, former Education Minister Yunus Qanooni (search), refused to concede defeat, raising the risk of political instability in a country slowly emerging from a quarter-century of war.

The election itself was delayed from June because of insecurity and logistical problems. Qanooni and other challengers claimed massive fraud in favor of Karzai and threatened to boycott the results.

In its final report released Wednesday, the board confirmed there were problems with ballot stuffing and with the ink used to mark people's fingers to prevent multiple voting.

But it said there was "no evidence" the problems were widespread or that they favored only Karzai.

"There were shortcomings," said Staffan Darnolf, a Swedish election expert on the panel. "But they could not have materially affected the overall result."

Qanooni's running mate, Syed Hussein Alemi Balkhi, said the report was "unacceptable," but he stopped short of saying the challengers would reject the election result.

"We had a lot of questions, but the panel was not able to answer them," Balkhi said. "We are not satisfied with their findings."

Qanooni won 16.3 percent of the vote, followed by Hazara chieftain Mohammed Mohaqeq (search) with 11.7 percent.

Mohaqeq, who vowed "never" to recognize Karzai's victory, declined to comment Wednesday.

British Prime Minister Tony Blair congratulated Karzai and said the elections were a "tremendous achievement for the Afghan people." He added that Britain and the international community would continue to work for the country's stability and democracy.

NATO chief Jaap de Hoop Scheffer (search) also sent his congratulations to Karzai.

"This was first and foremost a victory for the Afghan people," de Hoop Scheffer said in a statement, adding that he "looked forward to working" with Karzai "in helping Afghanistan build a better, safer future."

A French Foreign Ministry statement said Afghans "by their high participation rate and a clear choice, gave strong legitimacy to this vote and to the new president."

Karzai, who will be inaugurated this month, has vowed to accelerate the slow rebuilding of a country shattered by war and drought with the goal of doubling the income of ordinary Afghans by 2009.

But any attempt to focus on the economy will be complicated by the challenge of confronting warlords and drug traffickers even as a stubborn insurgency grinds on.

The size of his task — and the rancor surrounding the vote — also has been highlighted by an ongoing hostage crisis involving three foreign election workers.

The abductions last week were claimed by a splinter group of the Taliban, which had vowed to attack the election process, but officials also suspect the involvement of militia leaders, who could lose out if Karzai presses on with efforts to disarm unruly warlords.

Some 8 million Afghans voted more than three weeks ago, a turnout of 70 percent and a show of enthusiasm for a democratic experiment.

Thirteen election workers died in bombings and shootings in the run-up to the ballot. Tens of thousands of foreign and Afghan security forces were deployed on polling day, and helicopters and donkeys were used to bring ballot boxes from remote mountain villages for counting by election workers.

Karzai earned an overwhelming majority of votes among his fellow Pashtuns across the country's south and east, and he won in every major city.

Still, he lost out to rivals across a swathe of the north and center dominated by smaller ethnic groups, underlining the ethnic divides opened by years of bitter fighting.