KABUL -- British officials yesterday blamed a "rogue" Afghan policeman for the shooting deaths of five British soldiers at a checkpoint in the volatile south, raising concerns about the possible infiltration of local security forces by militants.

Zemeri Bashary, spokesman for the Afghan Interior Ministry, confirmed that the assailant was wearing a police uniform but said it was too soon to say whether he was a member of the force.

Tuesday's attack took place the day after Afghan electoral officials proclaimed President Hamid Karzai the winner of a disputed election following the withdrawal of his rival from a weekend runoff. Former Foreign Minister Abdullah Abdullah, who quit the race citing fears of fraud, said yesterday that he had no interest in joining Mr. Karzai's administration and would press for reforms from outside the government.

The latest deaths raised the number of British troops killed since the 2001 invasion to 229 and were likely to fuel debate about the country's continued involvement in the war. Britain is the second-largest contributor of forces to Afghanistan after the United States, with 9,000 troops.

Concerns have been raised before about the quality of the Afghan National Police, or ANP, some of whose members have been accused of drug trafficking, corruption and other abuses. The shooting happened a month after an Afghan policeman turned his gun on U.S. troops during a joint patrol in Wardak province, killing two of them.

"The problem could get more challenging as we try to accelerate recruiting, so we will have to think hard about finding ways to prevent future recurrences," Michael O'Hanlon, a security expert with Brookings Institution in Washington, said by e-mail. "It argues for steps like creating biometric databases and even more carefully screening applicants."

A key part of the strategy in Afghanistan is to build competent army and police forces that eventually can take over security responsibilities.

A British Defense Ministry spokesman said the dead soldiers, three from the Grenadier Guards and two from the Royal Military Police, were part of a team mentoring the police in Helmand province, a center of the Taliban insurgency in the south and the focus of a major U.S. offensive over the summer.

British Prime Minister Gordon Brown told Parliament that the Taliban had claimed responsibility for the attack.

"We will step up security," he said. "But we must not allow ourselves to give up what the Afghan Taliban fear most: that we will have a strong Afghan security force that is Afghan-based and is able to face them."

The attack took place in the afternoon, at a busy checkpoint in the Nad-e-Ali district. British and Afghan forces live and work together at the checkpoint and "are so friendly they didn't have any armored vests on," said Dawoud Ahmadi, a spokesman for the Helmand provincial government.

"We believe some form of rogue ANP officer has opened fire on our personnel while they were visiting" the checkpoint, the British Defense Ministry spokesman, speaking on customary condition of anonymity, said by telephone from London. "It is believed that the guy infiltrated the police unit."

Mr. Ahmadi blamed the incident on poor vetting of police recruits.

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