In the past month, 19 servicemen have been killed in Helmand
The UK is to send another 125 soldiers to Afghanistan to reinforce troop numbers amid rising casualty levels, the defence secretary has announced.
It is the first time that casualties have been replaced and follows calls from commanders on the ground for more troops and equipment.
UK forces are currently fighting a major offensive against insurgent strongholds in Helmand.
Nineteen UK servicemen have died in July, with 188 killed since 2001.
Defence Secretary Bob Ainsworth said: "I have always said that I will listen to the view of commanders on the ground in Afghanistan - they are the people best placed to know the resources needed for that operation.
'Sad and tragic'
"In this case they have told me that, after the sad and tragic casualty rate that we have suffered in recent weeks, reinforcements are necessary to ensure we can maintain our operational tempo and consolidate the real progress we have made.
"These additional troops will ensure we have sufficient troop levels and, crucially, the right specialist skills in theatre.
"Many of our brave young men have died fighting to protect our national interest in Afghanistan and I will not allow their sacrifices to have been in vain."
The Ministry of Defence said it would keep about 9,000 British troops in Afghanistan, up from 7,800 last year.
The deployment will include a company from 3rd Battalion The Yorkshire Regiment (Duke of Wellington's), specialist counter IED personnel from 11 Explosive Ordnance Disposal Regiment The Royal Logistic Corps and members of 19 Regiment Royal Artillery.
Prime Minister Gordon Brown spoke to US President Barack Obama on Friday evening about the need for more burden-sharing within the Nato alliance, according to Downing Street.
A spokesman said: "They agreed that the mission in Afghanistan remained critical to the national security of Afghanistan, the US and the UK.
"Both leaders paid tribute to the sacrifices made by Nato troops."
They had agreed that despite the losses, military operations in Helmand were making progress and helping to improve security ahead of the Afghan elections, he added.
Earlier, the chief of defence staff, Sir Jock Stirrup, appeared to contradict comments made last week when he said having more helicopters available in Afghanistan would make no difference to the current mission.
"They [helicopters] are very useful and they certainly provide much more tactical flexibility, they enable you to be unpredictable and therefore they make it harder for the enemy to target you with improvised explosive devices," he told BBC's World at One.
"But on operations such as this... you can't get through this defensive crust of improvised explosive devices with helicopters.
"You've got to fight your way through on foot. There is no alternative.
"Helicopters were used in this operation but more helicopters, had they been available, would have made no difference," he added.
However, last week he said additional helicopters would "quite patently" prevent casualties and appealed to the government for more.
A spokesman for the Ministry of Defence denied that Sir Jock was being contradictory.
"The point we are making is that we have got sufficient helicopter resources to do what we are doing at the moment.
"If we had more helicopters, we could make progress quicker and achieve more.
"We are working to get more helicopters into theatre. When we do, that will aid our operations."
He added that most of those whose lives were lost would not have been saved by more helicopters, as they were on foot patrol.
He went on to say that Sir Jock spoke to the prime minister on a weekly basis and was "always willing to be frank" with him.
Asked whether Sir Jock's comments were a sign he was reining in his criticism of the government, the spokesman said it was doing Sir Jock a disservice to suggest he could be "silenced".