After a week in which the number of UK troops killed in Afghanistan rose to 230, Gordon Brown has said that Britain cannot "walk away".
In a speech aimed at shoring up public support for the operation, the prime minister stressed that "we will succeed or fail together, and we will succeed".
He defended the "vital" role that Britain is playing in training Afghan troops, saying that it was not the "safest" or "easiest" thing to do, but that it was the "right strategy".
Brown pledged to do everything possible to protect UK troops, but insisted that the training of Afghan security forces provides the viable exit strategy.
"We know that al Qaeda continue to train and plot attacks on Britain from the region," he said.
"We cannot, must not and will not walk away. We will not be deterred, dissuaded or diverted from taking whatever measures are necessary to protect our security."
He said that a series of five tests had been agreed with President Karzai to improve the country's security, governance, political settlement, economic development and relations with neighbours.
If Karzai's government failed to fulfill these requirements then Brown argued it would forfeit its right to international support.
"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," he said.
Brown's speech follows earlier calls from Foreign Office minister Kim Howells for the withdrawal of troops from Afghanistan, with money spent instead on anti-terror measures in Britain.
