By Caroline Wyatt
Defence correspondent, BBC News
There are currently about 9,000 British personnel in Afghanistan
Gordon Brown has announced that an extra 500 British troops will be sent to Afghanistan - as long as certain conditions are met.
Commanders in Helmand province say they need the additional manpower to help hold ground and build on progress made over the summer when British forces took key areas from the Taliban during Operation Panther's Claw.
The increase in numbers may sound small, but military chiefs say the deployment will help strengthen and deepen security in those areas by "thickening" the British presence.
That should also help better protect British soldiers in places such as Sangin, which until now have been more exposed to attack - especially from IEDs, or improvised explosive devices.
Crucially, the extra forces being sent to Helmand will be mainly frontline combat troops.
And although the headline figure announced by the prime minister was 500, the full rise in the number of frontline personnel will be closer to 1,000.
That is because a British battlegroup currently deployed in Kandahar province under Nato international command will now be redeployed solely to Helmand under British control.
The PM told the House of Commons that Regional Battlegroup South would be transferred immediately "to meet the changing demands of the campaign".
The bulk of the personnel who will make up the extra 500 could be sent in a matter of weeks, although elements of the Royal Anglian Regiment and the 1st Battalion Coldstream Guards are expected to go even sooner than that.
Some believe there is a wider political reason for Mr Brown announcing this decision now - just as President Obama decides whether to send more American forces to Afghanistan.
The US and Nato commander in Kabul, General Stanley McChrystal, has recommended options including at least 40,000 more troops for the mission.
He has also warned President Obama and his administration that Nato has a limited period in which to turn around a deteriorating situation.
A decision from Washington is expected perhaps as early as next week, with Mr Brown hinting that Britain's decision would be "consistent" with US plans.
All this also puts pressure on Britain's other Nato allies in Europe, with an informal defence ministers' meeting in Bratislava next week expected to be another important step on the road to securing a wider troop uplift.
It may be a chance to encourage a renewed focus on putting maximum Nato effort into getting the mission right before it is too late - if, as commanders say, the alliance has a year or so to start making real progress.
In a statement, Air Chief Marshal Sir Jock Stirrup said the increase in force level would provide 11 Light Brigade with the resources they need to "sustain and build on the hard-won achievements of 19 Light Brigade over the summer - achievements of which the entire nation can and should be proud".
Meanwhile, head of the Army General Sir David Richards warned once again that Britain could not afford to fail in Afghanistan.
"We asked for 9,500 and that is what we have got," he said. "Having sufficient troops on the ground is the key to this campaign."