Gordon Brown today welcomed US President Barack Obama's long- awaited announcement of a 30,000-strong troop "surge" for Afghanistan.
The Prime Minister said a "very substantial part" of the additional US force would be deployed to Helmand province where British troops have been engaged in heavy fighting against the Taliban.
However, he played down suggestions that British troops would start withdrawing from Afghanistan next year as the Afghans begin taking responsibility for security for some provinces and districts.
"There was no question of us withdrawing our British troops until the point that we were sure that the Afghans could take over security control themselves," he told MPs at Prime Minister's Questions.
"We will stay and do the job that is necessary."
In his highly-anticipated address, Mr Obama pledged to bring the conflict to a "successful conclusion" by introducing the surge as quickly as possible.
But, while increasing the total American presence in the region to almost 100,000 by next summer, the President warned that the commitment would not be open-ended.
He set out plans for Afghan security forces to be trained to a sufficient level to enable the US to start pulling troops out as early as July 2011.
He also appealed to America's allies to step up their own commitments in the region, insisting: "This is not just America's war."
Mr Brown, who confirmed plans on Monday to send an additional 500 British troops, said that by the end of next year the total number of Afghan and coalition forces in the country will have risen from 200,000 to 300,000, enabling the Afghans to start taking control of some districts province.
"That is the point at which the balance between Afghan forces and British and American and coalition troops will start to change," he said.
"It is absolutely crucial for our Afghanisation strategy that the Afghans start to take control of the security as soon as possible.
"Even if one or two parts of a district or a province are transferred in 2010, we will continue to have our troops in Afghanistan at that point."
Mr Obama's announcement came more than three months after the White House received a report from the top US commander in Afghanistan, General Stanley McChrystal, recommending that 40,000 extra soldiers were needed.
In a speech to a military academy in West Point, New York State, the president indicated he had opted to go with fewer, but will deploy troops in a quicker time-frame that previously thought.
The accelerated schedule will see some additional troops in place by Christmas, it has been suggested, with the rest deployed by next summer.
The new strategy consists of three core elements: a renewed military effort; a civilian surge to reinforce positive action; and an effective partnership with Pakistan, Mr Obama said.
"These are the resources that we need to seize the initiative, while building the Afghan capacity that can allow for a responsible transition of our forces out of Afghanistan," he said.
"They will increase our ability to train competent Afghan security forces and to partner with them so that more Afghans can get into the fight.
"And they will help create the conditions for the United States to transfer responsibility to the Afghans."
He also pushed other members of the coalition to up their presence in an attempt to end the conflict, uphold Nato's credibility and bring security to the wider international community.
Mr Brown said that the London conference on Afghanistan that he has called for January 28 would be a "vital next stage".
"Britain will continue to play its full part in persuading other countries to offer troops to the Afghanistan campaign," he said.
Downing Street said it was not intended as a "troop-pledging conference" but would be a "very significant event".
"There has been a lot of talk about benchmarks and milestones and I am sure they will be discussed," the Prime Minister's spokesman said.Reuse content