They will be sent as long as they have the necessary equipment, if other Nato allies boost their troop numbers and more Afghan soldiers are trained.
He earlier paid tribute to each of the 37 British service personnel killed in Afghanistan since the end of July, among the 221 who have died since 2001.
There are currently about 9,000 UK troops stationed in Afghanistan.
There are also 150 reserve troops in the country which the Ministry of Defence said would be available for further temporary deployments.
'Solemn moment'
Mr Brown devoted two minutes at the start of prime minister's questions to reading out the names of all those who had died in Afghanistan since Parliament rose for the summer recess.
Describing it as a "solemn moment" for Parliament and the country, he said their sacrifices would not be forgotten.
While nothing could "erase the pain" of bereaved families, Mr Brown added that they could be proud of the actions of their loved ones and know that their "influence" would live on.
In a statement later, Mr Brown said he was prepared, in principle, to send an extra 500 troops to Helmand Province in southern Afghanistan - where Nato troops are fighting fierce battles with the Taliban - in response to military advice.
Mr Brown said the troop reinforcements - which would increase overall UK troop numbers to 9,500 - were conditional on three factors.
The prime minister said he needed to be sure the troops had the equipment and training necessary to perform the tasks asked of them.
He said two additional Merlin helicopters would be sent to Helmand in the near future and that more armoured vehicles and other specialist equipment would be provided to counter the threat of roadside bombs.
He also insisted Nato countries must bear their "fair share" of the mission by sending more troops of their own and the Afghan government had to be prepared to substantially increase their own troop presence in Helmand.
'Safer Britain'
Only if he received such guarantees, Mr Brown said, would he be able to "justify" sending more troops.
However, Mr Brown defended the UK's continued presence in Afghanistan, saying a "safer Afghanistan was a safer Britain".
"When the safety of our country is at stake, we cannot and we will not walk away," he said. "We have the right strategy and we will see it through."
Conservative leader David Cameron and Lib Dem leader Nick Clegg also paid tribute to those who had lost their lives.
However, they later raised issues with the prime minister about aspects of the Afghan war and support for British soldiers.
Mr Cameron said the military covenant with British troops needed to be "fundamentally re-examined" with more resources provided for helping badly injured troops with their rehabilitation.
He also said it was "totally unacceptable" some troops were not properly prepared for going to Afghanistan, mentioning the case of a Territorial Army soldier he said had told him he was not getting full training or pay.
Mr Brown said he was "determined" to ensure the armed forces received all the support they deserved and said he was confident that troops in Afghanistan were "properly resourced".
Lib Dem leader Nick Clegg welcomed the supply of more helicopters but said there must be be a "radical" change of strategy in Afghanistan with more emphasis on political reconciliation.
The UK has the second-largest contingent after the US.
The BBC's defence correspondent Caroline Wyatt said: "The prime minister will want assurances from military chiefs that the extra troops will be properly equipped.
"He'll also expect Britain's Nato partners to follow suit by offering more forces themselves."
Nato defence ministers are likely to discuss troop reinforcements at an informal meeting in Slovakia next week.
US President Barack Obama is currently considering a request by General Stanley McChrystal, the commander of international forces in Afghanistan, for up to 40,000 more international troops.
British forces have been in Afghanistan since October 2001, with more than two-thirds stationed in Helmand.
Military commanders have said extra troops are needed to build on the progress of this summer's Operation Panther's Claw, in which UK forces helped to expel insurgents from key areas but sustained heavy casualties in the process.
The question of troop numbers in Afghanistan has become highly politicised in recent months.
The ex-head of the Army, Sir Richard Dannatt, has claimed that No 10 turned down a request to send up to 2,000 more troops earlier this year - a suggestion Downing Street has denied.
Former defence secretary John Hutton said on Wednesday that it would have been "helpful" to have sent the extra troops six months ago.
Gordon Brown says the UK will send 500 more forces personnel to Afghanistan - but only if key conditions are met.
Over 40 countries contribute forces to the international mission in Afghanistan. Isaf has full details of current contributing nations.
SOURCE : BBC
He earlier paid tribute to each of the 37 British service personnel killed in Afghanistan since the end of July, among the 221 who have died since 2001.
There are currently about 9,000 UK troops stationed in Afghanistan.
There are also 150 reserve troops in the country which the Ministry of Defence said would be available for further temporary deployments.
'Solemn moment'
Mr Brown devoted two minutes at the start of prime minister's questions to reading out the names of all those who had died in Afghanistan since Parliament rose for the summer recess.
Describing it as a "solemn moment" for Parliament and the country, he said their sacrifices would not be forgotten.
While nothing could "erase the pain" of bereaved families, Mr Brown added that they could be proud of the actions of their loved ones and know that their "influence" would live on.
In a statement later, Mr Brown said he was prepared, in principle, to send an extra 500 troops to Helmand Province in southern Afghanistan - where Nato troops are fighting fierce battles with the Taliban - in response to military advice.
Mr Brown said the troop reinforcements - which would increase overall UK troop numbers to 9,500 - were conditional on three factors.
The prime minister said he needed to be sure the troops had the equipment and training necessary to perform the tasks asked of them.
He said two additional Merlin helicopters would be sent to Helmand in the near future and that more armoured vehicles and other specialist equipment would be provided to counter the threat of roadside bombs.
He also insisted Nato countries must bear their "fair share" of the mission by sending more troops of their own and the Afghan government had to be prepared to substantially increase their own troop presence in Helmand.
'Safer Britain'
Only if he received such guarantees, Mr Brown said, would he be able to "justify" sending more troops.
However, Mr Brown defended the UK's continued presence in Afghanistan, saying a "safer Afghanistan was a safer Britain".
"When the safety of our country is at stake, we cannot and we will not walk away," he said. "We have the right strategy and we will see it through."
Conservative leader David Cameron and Lib Dem leader Nick Clegg also paid tribute to those who had lost their lives.
However, they later raised issues with the prime minister about aspects of the Afghan war and support for British soldiers.
Mr Cameron said the military covenant with British troops needed to be "fundamentally re-examined" with more resources provided for helping badly injured troops with their rehabilitation.
He also said it was "totally unacceptable" some troops were not properly prepared for going to Afghanistan, mentioning the case of a Territorial Army soldier he said had told him he was not getting full training or pay.
Mr Brown said he was "determined" to ensure the armed forces received all the support they deserved and said he was confident that troops in Afghanistan were "properly resourced".
Lib Dem leader Nick Clegg welcomed the supply of more helicopters but said there must be be a "radical" change of strategy in Afghanistan with more emphasis on political reconciliation.
The UK has the second-largest contingent after the US.
The BBC's defence correspondent Caroline Wyatt said: "The prime minister will want assurances from military chiefs that the extra troops will be properly equipped.
"He'll also expect Britain's Nato partners to follow suit by offering more forces themselves."
Nato defence ministers are likely to discuss troop reinforcements at an informal meeting in Slovakia next week.
US President Barack Obama is currently considering a request by General Stanley McChrystal, the commander of international forces in Afghanistan, for up to 40,000 more international troops.
British forces have been in Afghanistan since October 2001, with more than two-thirds stationed in Helmand.
Military commanders have said extra troops are needed to build on the progress of this summer's Operation Panther's Claw, in which UK forces helped to expel insurgents from key areas but sustained heavy casualties in the process.
The question of troop numbers in Afghanistan has become highly politicised in recent months.
The ex-head of the Army, Sir Richard Dannatt, has claimed that No 10 turned down a request to send up to 2,000 more troops earlier this year - a suggestion Downing Street has denied.
Former defence secretary John Hutton said on Wednesday that it would have been "helpful" to have sent the extra troops six months ago.
Gordon Brown says the UK will send 500 more forces personnel to Afghanistan - but only if key conditions are met.
Over 40 countries contribute forces to the international mission in Afghanistan. Isaf has full details of current contributing nations.
SOURCE : BBC