The Prime Minister announced today that Britain's total military effort in Afghanistan will increase to more than 10,000 troops.
In a detailed Commons statement, Mr Brown confirmed that all the conditions had been met to allow an extra 500 troops to be deployed in December - taking the force level to 9,500.
But he also disclosed that when special forces were included, the "total military effort" in Afghanistan will be in excess of 10,000 troops.
The statement comes just a day before US President Barack Obama, after months of deliberation, is expected to announce that he will send up to 35,000 more US Forces.
Mr Brown said the three conditions for increasing British military manpower - that they would be properly equipped, that coalition partners would also put in extra troops and that the Afghan Government would boost its own security effort - had now been met.
He told MPs that the "military surge" would be complemented by a "political surge" with more Afghan police, a police reform plan and more effective and accountable local administration in Afghanistan.
The Government would be "failing in our duty" if it did not work with coalition partners to counter the threat posed by the Taliban and al-Qa'ida, and help ensure a "safer Britain".
Mr Brown hailed British forces as the "best in the world" but acknowledged they had suffered "heavy and tragic" losses in one of the longest military campaigns of recent times.
He acknowledged that some argued it was better to counter the terrorist threat by defending "fortress Britain" and others asked why British troops were in Afghanistan at all.
"But as long as the Afghan/Pakistan border areas are the location of choice for al Qaida and the epicentre of global terrorism, it is the Government's judgment that we must address the terrorist threat at its source.
"As long as three quarters of the most serious terrorist plots against Britain have links to those Pakistan/Afghan border areas, we should be failing in our duty if we didn't work with our allies to deal with the problem where it starts."
It was essential that progress in driving al Qaida from Afghanistan must be matched by actions not just to isolate, but defeat, the terrorist group in Pakistan.
"The safety of people on the streets of Britain requires us to deny al Qaida the space to operate across Pakistan and deny them the option of returning to operate in Afghanistan."
Mr Brown said the first districts and provinces could "potentially" be handed over to Afghan control during next year, depending on them being ready to do so with more trained troops and better policing.
He announced that the London conference on Afghanistan would be held on January 28 with Afghan President Karzai and the secretary general of the UN attending "to unite the international community behind a programme now, and for the longer term, to help the Afghans secure and govern their own country".
There should be a "military surge, yes, but complimented by a political surge, that is most of all an Afghan surge".
Over the coming year the coalition was seeking a major expansion of the Afghan army from 90,000 to 134,000.
Increasingly it would be for Afghan forces to clear and hold ground as they prepared for the time when they could assume sole responsibility.
Mr Brown recalled that he had pledged the rise in British troops would only take place when he had been assured they would be fully equipped.
"At this morning's meeting of our Afghanistan and Pakistan national security committee, the chief of the defence staff gave that assurance that this condition had been met, both for the existing force and the additional 500 troops."
Newly arrived Merlin helicopters had today been given the "green light" for operations, a month ahead of schedule.
"Compared with three years ago, we have doubled helicopter flying hours. In the coming months these will increase by a further 20 per cent."
"There are almost twice as many Mastiff as there was a few months ago, which is about the strongest vehicle we've got in terms of mine protection. There is about a 75 per cent uplift in Ridgback, which is a slightly smaller version of the Mastiff but with the same level of protection.
"The package of protective mobility vehicles that we bought some time ago are now being flown into theatre in significant numbers."Reuse content