Gordon Brown today gave the go ahead "in principle" for the deployment of an additional 500 British troops to Afghanistan, taking the total UK force in the country to 9,500.
In a Commons statement, the Prime Minister said he was responding to "clear military advice" from the Chiefs of Staff and from military commanders on the ground.
However, he told MPs the reinforcements were dependent on a series of conditions being met - including a requirement that all troops would be fully equipped for the tasks they were being sent to undertake.
Earlier, he came under fire from former defence secretary John Hutton who said it would have been "much more helpful" if the reinforcements had been sent six months ago.
Mr Hutton, who resigned in May at a time when military chiefs were reported to be pressing for up to 2,000 additional troops, warned the Government could "screw it up really badly" if the forces were not given the resources they needed.
He told BBC2's Daily Politics programme: "If this is a mission about national security then you do everything that you need to do to secure it. No ifs and buts, because you've got to prosecute these campaigns absolutely clearly and with the force levels that you need to succeed.
"If you do an economy of force operation here I think you could screw it up really badly."
While the head of the Army, General Sir David Richards said he was "confident" that Mr Brown's criteria would be met, Downing Street was unable to say when the deployment of the additional troops would take place.
As well as the need to ensure they were properly equipped, Mr Brown said there had to be an agreed approach across the international coalition, with all countries, "bearing their fair share".
And with the outcome of the disputed presidential elections still to be decided, he said the next Afghan government needed to demonstrate its willingness to commit more troops to the fight alongside the coalition.
The Prime Minister's spokesman said Mr Brown hoped the conditions could be met "as quickly as possible", with a result possible in the Afghan elections "in the next few days".
Mr Brown said he had received assurances form both President Hamid Karzai and his main challenger, Dr Abdullah Abdullah, about the approach they would take in government.
Mr Brown told MPs it remained essential to stabilise the region if al Qaida was to be denied a base to mount fresh attacks on the West.
"If we limit ourselves simply to targeting al Qaida - without building the capacity of Afghanistan and Pakistan to deal with terrorism and violent extremism - the security gains will not endure," he said.
The Chief of the Defence Staff, Air Chief Marshal Sir Jock Stirrup, said they now needed to hear from the United States, where the administration of President Barack Obama is reviewing its Afghan strategy.
General Stanley McChrystal, the commander of international forces in the country, has put forward a proposal for up to 40,000 additional troops as part of a fullscale counter-insurgency strategy to build security across Afghanistan.
However others in the administration, including Vice-President Joe Biden, are reported to favour a more limited counter-terrorism strategy using unmanned aerial drones and special forces' raids to hit al Qaida targets.
Air Chief Marshal Stirrup signalled his strong support for Gen McChrystal's approach, saying he agreed with "just about all" of his report.
"The US administration has to decide what it wants to do. Naturally, what we do depends to a very (great) extent upon the decision of the United States and their commitment to the strategy and to the mission," he said.
"Not unreasonably, we need to await the outcome of that. I wouldn't want to put words into the mouths of the Americans, but I am fairly confident of the way that it is going to come out."
In the meantime, Mr Brown said, there would be a greater concentration of British forces in central Helmand, with the redeployment of the southern regional battle group from Kandahar.
It is understood the Royal Anglian Regiment is to be deployed to Helmand fulfil the role, ahead of the return next month of the Black Watch battle group from Kandahar.
In the Commons, Tory leader David Cameron called for Whitehall to be put on a "proper war footing" to run the Afghanistan campaign.
"We need a clear sense of direction from ministers, a clear sense of who is in day to day charge, and a government machine that responds quickly and decisively," he said.
Liberal Democrat leader Nick Clegg said there was still no overarching strategy and no coordinated international plan for Afghanistan.
"We cannot continue to fight this war on half-horsepower with half-measures and half-baked thinking," he said.
Earlier, Mr Brown began the first Prime Minister's questions of the new parliamentary term by reading out the roll-call of 37 British military personnel killed over the summer.
"This is a solemn moment for this House and our country," he said.
"It is a day on which we put on record our gratitude and our commemoration of the sacrifice made by 37 of our armed forces serving our country in Afghanistan."