The Prime Minister used a major policy speech to propose bringing forward a key Nato target to expand the Afghan army to 134,000 troops from 2011 to next year.
But he made no reference to last night's shock protest resignation of Labour MP Eric Joyce, who had been an aide to Defence Secretary Bob Ainsworth.
Meanwhile, the bodies of Sergeant Stuart Millar, 40, and Private Kevin Elliott, 24, who were both killed in Helmand province in southern Afghanistan on Monday, were flown home today.
Addressing the International Institute for Strategic Studies in London, Mr Brown insisted the military campaign was crucial to protect the public in this country and said the UK could not simply "walk away".
He did not set out a timetable for pulling out British troops, but suggested that transferring responsibility for security to the Afghans would allow the reduction of UK forces.
"I think the issue is how fast you can move on this. What we are saying today is that we are going to move faster," he said.
Nato announced in the spring that it would support the expansion of the Afghan army from 80,000 to 134,000 by November 2011.
Mr Brown said Britain would welcome a more ambitious target of achieving the goal of 134,000 by November 2010, which would require increasing the rate of training from 2,000 to 4,000 new troops per month.
To achieve this he proposed a shift from mentoring Afghan forces to partnering them, meaning that the bulk of British combat troops would eat, sleep and fight side-by-side with their Afghan counterparts.
He said: "If, as I say, Afghan forces can take more responsibility for the functions of security in the different parts of Afghanistan, and if perhaps we consider transfers of responsibility of government district by district or province by province, then it is possible to envisage that, as the number of Afghans taking responsibility grows and the quality of their leadership grows, we can reduce the numbers of our forces.
"That is the basis of our strategy and it is the basis of the American strategy as well. I can be time-specific about the build-up of Afghan forces that we are trying to achieve.
"When we see the quality of the effort, and when we are assured responsibility can be taken, that is the point at which we can say the need for British troops is a great deal less."
Last month the commander of British forces in Helmand said military control could be handed over to Afghan forces in parts of the province immediately.
Brigadier Tim Radford, commander of Task Force Helmand, suggested this could happen in the provincial capital, Lashkar Gah, and to a lesser extent in Garmsir.
But Liberal Democrat leader Nick Clegg said today it was "hard to believe" that training more Afghan forces would do the trick now after eight years of an "over-ambitious and under-resourced strategy".
He said: "We need a bolder change of strategy to turn things around. When it comes to Afghanistan, we need to do things properly or not at all."
Mr Joyce, a former Army major, resigned as Mr Ainsworth's parliamentary private secretary last night and launched a stinging attack on Government policy in Afghanistan.
He warned that the Government could no longer simply claim the conflict was a fight to combat terrorism, telling ITV News: "We have to be honest about what our commitment is.
"It's very high, and I don't think we can simply say the simple statement that we're preventing terrorism back in the UK.
"In many ways that's true but it's not enough of an explanation. People really need to know what would happen if we weren't there - would the Americans simply fill the gap?"
Mr Brown gave a flavour of the challenges faced by British troops in Afghanistan when he revealed that they have now found more than 1,000 improvised explosive devices (IEDs) during the current six-month tour.
A roadside bomb claimed the life of Lance Corporal Richard Brandon, 24, of the Corps of Royal Electrical and Mechanical Engineers, in Helmand on Wednesday.
His fiancee, Emma-Jayne, today described the father of one and stepfather of two as "one in a million".
Hundreds of people turned out today to pay their respects to Sgt Millar, from Inverness, and Pte Elliott, from Dundee, both from The Black Watch, 3rd Battalion The Royal Regiment of Scotland.
After a private chapel ceremony for their families at RAF Lyneham in Wiltshire, hearses carrying their Union Flag-draped coffins were driven in a memorial procession through the high street of nearby Wootton Bassett.
Among those lining the route was Colour Sergeant Gerry McQuade, 32, of the Royal Highland Fusiliers, 2nd Battalion The Royal Regiment of Scotland, a close friend of Sgt Millar.
He said: "We all understand the consequences of operational tours, that in a worst case scenario someone could die, but when it's someone you know it does really hit home. It's upset me more than it has in the past."
Meanwhile, a British soldier facing court martial for refusing to fight in Afghanistan had charges of disobeying a lawful command dropped today.
Lance Corporal Joe Glenton, 27, of the Royal Logistic Corps, now faces one charge of desertion for refusing to return to Afghanistan.
It was also revealed today that the Ministry of Defence faces paying wounded troops an extra £150 million if it loses a controversial legal challenge to the compensation paid to two injured servicemen.
Shadow defence secretary Liam Fox described the Prime Minister's speech as "underwhelming and short on detail".
"What will happen to the 900 troops that we sent specifically for the elections?" asked Dr Fox.
"If we are to speed up the training of the Afghan National Army, how exactly will this be done? Will it require more British troops?
"When his defence chiefs advised this course a few months ago, why did he reject their advice?
"On the equipment front, we need to see more armoured vehicles delivered more quickly and we need to know how they are going to be transported given the lack of airlift capacity. Much more detail is required about the practicalities on both manning and equipment."
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