President Karzai’s opponent withdrew from Afghanistan’s presidential election yesterday, effectively conceding defeat six days before a planned run-off but threatening to undermine the legitimacy of a new government.

Abdullah Abdullah stopped short of calling for his supporters to boycott Saturday’s vote and urged them not to take to the streets in protest, leaving a window open for a power-sharing deal that UN and American officials are trying to broker.

His decision still threw the process into confusion, with election officials insisting that the run-off should go ahead with only one candidate, while Western diplomats pressed for it to be cancelled because of the risk of a low turnout and Taleban attacks.

The move appeared to be designed to increase international pressure on Mr Karzai to strike a deal with Dr Abdullah as the only way to achieve a credible result from Afghanistan’s two-month election crisis.

Dr Abdullah announced his withdrawal at a meeting of several hundred supporters, including tribal elders and former Mujahidin commanders, mostly from the ethnic Tajik-dominated north of Afghanistan.

“In order to look God in the eye at this historic moment, I have made a decision,” he told the meeting in a giant tent in Kabul.

“In protest against the illegal actions of the Government and the electoral commission, I will not participate in the run-off,” he said, prompting cheers from the audience. “It hasn’t been easy for me to make this decision,” he said, his voice faltering, and his eyes welling with tears.

Asked later whether he was asking his supporters to boycott the vote, Dr Abdullah told reporters: “I have not made that call.”

Dr Abdullah’s campaign team had threatened to boycott the run-off after Mr Karzai refused to meet several “minimum conditions” to prevent fraud, including sacking the election chief and suspending three ministers, by October 31.

A total boycott would have challenged the legitimacy of the entire process, which the United States and its allies are desperately hoping will produce a credible government with which they can work to defeat the Taleban. It might also have prompted Dr Abdullah’s supporters to come on to the streets in protests that could easily become violent, and split the country between the Tajiks in the north and the ethnic Pashtuns in the south, who mostly support Mr Karzai.

Dr Abdullah therefore came under huge international pressure in the past few days to withdraw gracefully without challenging the legitimacy of the election — in exchange for a power-sharing deal with President Karzai. Kai Eide, the UN chief in Afghanistan, and Karl Eikenberry, the US Ambassador, almost brokered a deal yesterday morning whereby the two men would have divided up ministries and provincial governorships, diplomats said.

“They were so close to a deal, but at the last minute something happened,” one diplomat said. “As an olive branch to the internationals, Abdullah didn’t boycott.” At a news conference after his speech, Dr Abdullah admitted meeting President Karzai on Wednesday, but denied having struck a deal.

“This is my decision. This has not been made in exchange for anything with anybody,” he said. He conceded, however, that the door was still open for negotiations with Mr Karzai.

Western officials publicly welcomed his decision to withdraw, rather than boycott, describing it as “statesmanlike”. Gordon Brown said that Dr Abdullah had pulled out “in the interests of national unity”.

Mr Brown also said that he had spoken with Mr Karzai by telephone yesterday and the President had told him that he wanted to issue a “unity manifesto” for the Afghan people.

“The issue for me is what is to happen to Afghanistan in the future, given that our soldiers have contributed so much to the development of Afghan society,” Mr Brown said. “What is going to happen is we have got to have a visible sign that the new administration is tackling corruption.”

David Axelrod, President Obama’s senior adviser, suggested Dr Abdullah’s announcement was motivated more by common sense than despair. “I thought that his remarks today were rather moderate,” Mr Axelrod said. “He left open as to whether his supporters should participate in the process, and he is establishing himself as a leader of the Opposition.”

Mr Axelrod added that Dr Abdullah knew from polls that he was almost certain to be defeated, and that Washington was facing up to the same reality. “We are going to deal with the Government that is there,” he said. “Obviously there are issues we need to discuss, such as reducing the high level of corruption. These are issues we’ll take up with President Karzai.”

Hillary Clinton, the US Secretary of State, paid tribute to Dr Abdullah’s “dignified and constructive” campaign. “We hope that he will continue to stay engaged in the national dialogue and work on behalf of the security and prosperity of the people of Afghanistan,” she said.

Behind the scenes, Western diplomats continued their efforts to broker a deal that would allow the run-off to be cancelled. Mr Karzai’s office said that he “regretted” Dr Abdullah’s decision, but would abide by any ruling on the run-off from the Independent Election Commission or the courts.

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