Mr Browne said the security situation had improved, but tactics had changed
British troop numbers in Afghanistan will increase by 230 to a new high of more than 8,000 by next spring, Defence Secretary Des Browne has told MPs.
The new troops will boost protection for UK personnel, improve Afghan security training and aid reconstruction work.
Mr Browne said security had improved but the Taleban had changed tactics.
The number of British soldiers killed in Afghanistan since 2001 reached 102 last week.
British casualties have crept up and are now second only to the US in the number of fatalities.
Furthermore, the casualty rates for Britons serving in Afghanistan are now higher than those in Iraq.
There are currently 53,000 Nato troops in Afghanistan. The announcement takes the UK's commitment from 7,800 troops - based in Kabul, Kandahar and Helmand - to 8,030.
Mr Browne said 630 new posts were being created but, with 400 administrative and support posts to be closed, the net increase in personnel would be 230.
The additional troops will be specialists, such as helicopter crews, engineers and maintenance staff, with the aim of refocusing efforts on reconstruction and the training of the Afghan police and army.
Mr Browne said the Taleban had switched from insurgency to terrorist tactics - including suicide bombers recruited from "vulnerable" communities.
But he said, despite setbacks like the recent outbreak of Taleban fighters from a Kandahar prison: "Our view is that the Taleban are losing the fight in southern Afghanistan."
The defence secretary said the new deployments included soldiers to man extra Viking and Mastiff vehicles, more specialists for reconnaissance and warning systems in Helmand, and reinforcements at the Royal Air Force Regiment Squadron at Kandahar airbase.
Security... remains undermined in the south by the continued insurgency, the breakout of Taleban prisoners in Kandahar and the attitude of the Pakistani government about their approach to border issues
He added that when 3 Commando Brigade was deployed in October it would have an additional infantry battalion headquarters and there would be an extra troop of Royal Engineers to assist with projects to support local communities.
Mr Browne also said that the Harrier force was being withdrawn from the country - having first been deployed to Kandahar airfield in November 2004 - as he was "very mindful of the strain" that the extended deployment had put upon the crews.
They will be withdrawn by next spring and replaced with Tornado GR4s.
Shadow defence secretary Liam Fox said the US, Britain, Canada, Holland and some other smaller forces were bearing the brunt of fighting in southern Afghanistan - and accused other Nato allies of being "risk averse".
He also said progress had been slow in some areas - particularly on offering alternative source of income to those who grow poppies.
The prime minister announces more troops will go to Afghanistan
He added: "Security has improved in some parts of the country as the secretary of state has just said, but it remains undermined in the south by the continued insurgency, the breakout of Taleban prisoners in Kandahar and the attitude of the Pakistani government about their approach to border issues."
For the Liberal Democrats, Nick Harvey said he welcomed the statement - particularly extra engineers posts to concentrate on the civilian reconstruction work and an increase in helicopter crew members.
But he said his biggest concern was the "state of British public opinion" which he was concerned did not understand what troops were doing in Afghanistan and how long it would take.
He said it was unfortunate the statement had been made on the day of President Bush's visit - "I fear there is a confusion in the public mind between what we are doing in Afghanistan and Iraq.
It is in the British national interest to confront the Taleban in Afghanistan
"The biggest connection between the two is that of overstretch and the sooner the public can be persuaded to see these things quite separately the better our chances ... of getting the public on board for the long haul in Afghanistan."
Earlier, Prime Minister Gordon Brown paid tribute to bravery of UK personnel in Afghanistan.
And on the rising troop numbers, he said: "We have resolved, first of all, as we did some years ago, that it is in the British national interest to confront the Taleban in Afghanistan or Afghanistan would come to us."
But he said it was not a question of moving British troops from Iraq to Afghanistan as there was a job to do in both: "You can't trade numbers between the two countries."
He said Afghanistan had been one of the main topics of conversation with President Bush, who has spent two days visiting the UK.
He said President Bush was "a true friend of Britain" and thanked him for "the importance he attaches to enhancing our transatlantic partnership from the work we do in Afghanistan and Iraq to every part of the world".
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