President Karzai has six months to sideline his brother and reduce corruption or risk losing American support, Afghan officials have told The Times of London.

Senior palace insiders said that President Obama delivered the ultimatum when he congratulated Karzai on his re-election on Monday. Top of his demands was action against corruption, the appointment of "reform-minded ministers" and several high-profile scalps to prove Karzai's commitment to cleaning up his government.

"If he doesn't meet the conditions within six months, Obama has told him America will pull out," said an official with access to Karzai's inner circle. "Obama said they don't want their soldiers' lives wasted for nothing. They want changes in Cabinet, and changes in his personal staff."

It is extremely unlikely that British troops would stay in Afghanistan if U.S. forces were withdrawn.

The president's half-brother, Ahmed Wali Karzai, has repeatedly denied claims that he controls Afghanistan's billion-dollar heroin trade. As head of Kandahar's provincial council, he is the main powerbroker in the south of the country, but the president has refused to remove him, insisting that there is no proof of wrongdoing.

In his acceptance speech yesterday, Karzai vowed to eradicate the "dark stain of corruption", which he admitted had undermined faith in his regime. But Obama had earlier cautioned: "The proof is not going to be in words; it's going to be in deeds."

Afghan officials said that there were efforts to find Wali Karzai a new position. "There have been talks to find a new term of reference for the president's brother," said one. One option would be to send him abroad.

The American Embassy is understood to have warned Karzai it will start collecting evidence against Wali Karzai if he is not removed from Kandahar. "They want tangible progress over the next three to six months on corruption, the culture of impunity and rising crime," said a senior Western analyst with close links to the State Department.

Continue reading at The Times of London