Britain is to increase its military presence in southern Afghanistan by about 800 troops to 5,800.
The number of British troops in Afghanistan is set to rise
Defence Secretary Des Browne, who made the announcement, said the extra troops would be in place by late summer.
But the UK's overall deployment in Afghanistan will only increase by 300 because the military is also reducing its presence in Kabul by 500 personnel.
The British troops are part of a 32,000-strong Nato force which is currently based in Afghanistan.
Britain's presence in the capital Kabul is to be reduced this weekend as it hands over command of the Nato-led International Security Assistance Force to the US.
Britain will still have 140 British personnel stationed in Kabul to assist the ISAF.
The change in troop numbers come as 3 Commando Brigade of the Royal Marines prepares to end its tour of duty in Helmand in April.
Mr Browne made the troop announcement in a written statement to MPs, in which he also confirmed that 3 Commando will be replaced by units drawn mainly from 12 Mechanised Brigade.
He also said that Harrier GR7 and GR9 jets, Apache attack helicopters, Viking all-terrain vehicles and Royal Engineer support units which are currently stationed in Helmand will remain there until April 2009.
Around 600 call-out notices are to be served on reservists, which will lead to around 420 posts being taken up.
Former Nato Secretary-General Lord Carrington has previously accused France and Germany of "not pulling their weight" by providing troops for the south of the country, where the Taleban still has control over large areas.
Mr Browne's announcement comes as the US presented its first consignment of new military equipment to the Afghan army.
The consignment includes more than 200 Humvee armoured vehicles, 800 heavy army trucks and thousands of rifles.
The BBC's Alastair Leithead said many other countries do have troops in Afghanistan but also have "rules and regulations" which prevent them from going into the south, where the bulk of the fighting takes place.
Shadow defence secretary Liam Fox said there were "serious questions" about why Britain was taking an increased role when other countries in Nato were not doing the same.
"There have to be serious questions raised about why the British Army are shouldering yet more of the burden down in the south of Afghanistan," he said.
"It cannot be acceptable that British taxpayers are funding a greater proportion of the cost and the British military are shouldering a greater part of the burden in the most dangerous part of the country.
"Where are our Nato allies? This is simply not an acceptable long term position.
"It is absolutely outrageous that when we have the concept of shared security we don't have properly shared risk."
Lib Dem defence secretary, Nick Harvey, said: "There is a real danger of our troops suffering from a mismatch between the commitments we are taking on around the world and the resources available to us.
"By setting up a framework to withdraw our troops from Iraq we would be able to sustain our presence in Afghanistan without leaving British soldiers suffering from the effects of overstretch."