It will bring Britain’s total deployment in southern Afghanistan to about 4,400.
Britain’s Defence Secretary Des Browne also said that more support helicopters would be sent to the troubled region together with a radar installation.
British forces in southern Afghanistan are concentrated in the troubled Helmand province, where six British soldiers have died in Taliban-related combat in the past month.
The extra troops will be drawn from the Royal Marines, the Royal Regiment of Fusiliers, the Royal Irish Regiment and the Royal Engineers, with the latter to "accelerate the reconstruction effort".
Military personnel
Senior military personnel said a force of about 150 men from the Royal Regiment of Fusiliers - currently on standby in Cyprus, would be sent in the next couple days.
Two platoons, about 60 soldiers, from the Royal Irish Regiment will follow in the coming weeks while a 300-strong Royal Engineers contingent will support British government-backed aid and reconstruction work.
Other personnel will be made up of headquarters staff, intelligence, logistics, medics and air support teams plus a 40-strong group from 45 Commando to help train the Afghan army.
One commander insisted that the deaths of the six soldiers did not prompt the decision to send extra troops, instead explaining that it was part of a continual reassessment of capacity on the ground.
Prime Minister Tony Blair said that Britain's mission to bring security to southern Afghanistan was going to be tough because troops were deploying in a particularly troubled region for the first time.
Britain is due to head a NATO-led force in southern Afghanistan from the end of July that includes Australian forces, taking over control of security from the US military.
The mission aims to rid the area of Taliban violence, rebuild the economy and replace widespread opium farming with alternative livelihoods.
British forces stretched
However senior British military sources have told AFP that the extra troops will stretch defence force’s overall capacity.
The sources admitted the situation left Britain’s armed forces "stretched, but not over-stretched", while Defence Secretary Des Browne described the workload as "challenging, but sustainable".
"There's no doubt that these two theatres (Iraq and Afghanistan) are stretching us," said one high-ranking military commander.
But he insisted: "The frontline commanders are clear they can cope with this and manage this. These are critical campaigns that we have to see through."
He also pointed out that the pressure on British troops would decrease as Iraqi and Afghan security forces are trained and deployed.