Britain today warned Afghan President Hamid Karzai that he risked the withdrawal of international support if his government fails to improve security and root out corruption, dpa reported. "I am not prepared to put the lives of British men and women in harm"s way for a government that does not stand up against corruption," Prime Minister Gordon Brown said in a speech in London. "Sadly, the government of Afghanistan has become a by-word for corruption," added Brown. International support for Afghanistan depended on five key areas - security, governance, reconciliation, economic development and engagement with its neighbours, including Pakistan, Brown said. "If the government fails to meet these five tests, it will not only have failed its own people, it will have forfeited its right to international support." Commentators said Brown"s speech was the clearest warning yet by Britain that the mission in Afghanistan could fail. But Brown, speaking after the shock murder of five British soldiers by an Afghan policeman in southern Helmand, again underlined the importance of the Afghan mission in protecting Britain and other nations from terrorism. Britain could "not afford to walk away" from the mission in Afghanistan, Brown said in a hastily arranged speech at the Royal College of Defence Studies in London. Recent opinion polls have shown a drop in popular support in Britain for the Afghan mission, in which 230 British soldiers have died since 2001. Of those deaths, 92 occurred this year, making 2009 the "bloodiest" year so far for Britain in the Afghan conflict. Britain lost 179 soldiers in the Iraq war. An opinion poll published this week showed that opposition to the war in Afghanistan has risen sharply in Britain, with Brown apparently unable to "galvanize support" for troop deployment. According to the YouGov survey, 35 per cent of Britons believe all troops should be withdrawn, compared with 25 per cent backing that call just two weeks ago. Meanwhile, for the first time, small wooden crosses bearing pictures of the overwhelmingly young victims of the wars in Iraq and Afghanistan have been added to the Field of Remembrance outside Westminster Abbey in London where 60,000 crosses are erected each year to remember the war dead.