A small team of British officers are in Kabul
Tony Blair has said he expects British peacekeepers to arrive in Kabul by the end of the week.
But complete details of the international peacekeeping force for Afghanistan are still being worked out.
The country's interim government, a product of recent talks in Bonn, is due to take office on Saturday.
Lead role
Mr Blair spoke of the "urgent need to ensure that as the war is being won, we play our part in securing the peace," adding that Britain's eventual contribution could total between 1,000 and 1,500.
"The fact is we are the nation best placed to give that leadership," he told MPs, "which is why we've been asked to do it."
Several European and Moslem countries are expecting to take part, as well as Canada, Australia and New Zealand.
Mr Blair said the United States, still heavily engaged in fighting the remnants of al-Qaeda and the Taleban, had offered its full support.
Without American logistics, transport capability and air cover, British officials admit a peacekeeping operation could not be mounted.
British lead elements will be on the ground by the end of the week.
Defence officials say these will be Royal Marines from 40 Commando, based in Taunton.
They are part of a task force that has been stationed in the Indian Ocean for several weeks.
Team report
But it may be some time before any other troops, British, European or anyone else, fly out to join them.
A small liaison team, led by Major General John McColl, is still in Kabul, attempting to resolve a number of issues, including the size of the peacekeeping force.
Afghanistan's incoming defence minister, Mohammed Fahim, initially said he wanted a maximum of 1,000 soldiers to protect the new government.
Mr Blair's reference to as many as 1,500 British troops alone suggests, say defence officials, that progress has been made on this issue.
The total force is likely to number 3-5,000, depending on the exact nature of the mission and whether peacekeepers will be permitted to patrol outside Kabul.
General McColl is likely to report back as early as Tuesday, with a further announcement on troops now expected on Wednesday.
Tory leader Iain Duncan Smith expressed "deep misgivings" about the mission, warning British troops could find themselves the target of retaliation by remnants of the Taleban.
Mr Blair refused to put a time limit on the operation, saying only that it would last "several months."