He stressed that a timetable for withdrawing British forces from the increasingly unpopular war would only be drafted once the Afghan army and police show they can maintain security.
He plans to build up Afghan forces to 50,000, and committed Britain to training 5,000 in Helmand province alone by the end of 2010.
The British leader also called for 5,000 more troops from other countries outside of Britain and the United States to be deployed with NATO's International Security Assistance Force (ISAF).
Brown made it clear the international community expected Hamid Karzai, the Afghan president , who has been slipping from global favour among widespread allegations of corruption, to step up and assume his responsibilities.
Karzai has to realise "that there will be milestones by which he's going to be judged and he's got to accept that there will be benchmarks which the international community will set," Brown said.
Karzai, re-elected to a second term after fraud-tainted elections, is coming under growing pressure to prove his government is a reliable partner as the conflict bogs down and the death toll among US and NATO forces mounts.
The London and Kabul talks will "outline the framework for an increased lead role for the Afghans in the shaping of their destiny," Ban Ki-moon, the UN secretary-general, who appeared with Brown, told reporters.
The conference, which has also been backed by Angela Merkel, the German Chancellor, won swift support from Washington which has 68,000 troops in Afghanistan.
Barack Obama the US president, is to unveil a new strategy on Afghanistan on Tuesday. He is expected to commit more than 30,000 fresh US troops to the conflict.
But Obama, who has vowed to "finish the job" in Afghanistan, will also lay out an exit strategy to withdraw from a conflict now in its ninth year.
At least 483 foreign troops, about half of them Americans and some 98 Britons, have been killed in Afghanistan this year, according to the icasualties.org website, making it one of the deadliest years for troops since the US-led invasion in 2001.