Nearly 900 extra troops are to be sent to Afghanistan, Defence Secretary Des Browne has announced.
There are around 4,000 UK troops stationed in Afghanistan
He told MPs the reinforcements, which will boost troop levels to 4,500, will head for the Helmand province to help security and reconstruction efforts.
The extra deployment will include more than 300 engineers. More helicopters will also be provided.
Extra troops were requested for the region following the deaths of six British soldiers in the past month.
In a Commons statement, Mr Browne said 320 engineers from 28 Regiment Royal Engineers would be used to "accelerate the reconstruction effort".
A company from 3 Commando Brigade Royal Marines would provide force protection for them and those deployments would take place from September.
An extra infantry company, taken from the 2nd Battalion The Royal Regiment of Fusiliers, will also provide more mobile forces.
And two platoons from the 1st Battalion, The Royal Irish Regiment would provide additional force protection.
The defence secretary said there would also be a "small increase" in HQ staff and medical and logistical support.
And 450 reservists were being called up to fill 400 posts "in theatre".
"These enhancements, some 845 personnel, will place additional demands on our air transport," said Mr Browne.
"We have already increased the flying hours available for attack and support helicopters, as requested by commanders.
"And today I can say we will also be making more support helicopters available. We also plan to install a radar installation, provided by No 1 Air Control Centre, RAF."
Mr Browne told BBC News the need for extra troops did not mean the government had underestimated the task for soldiers, instead it was because of troops' success.
"Our commanders on the ground discovered there were opportunities beyond the area we had first planned to deliver security," he said.
The commanders had seen the chance to reinforce the position of the local governor and Afghan army and police by going into northern Helmand, he explained.
Mr Browne told MPs some extra troops would be going immediately, others in October.
Some soldiers would be replacing others but the announcement meant the "steady-state size" of the Helmand Taskforce would increase from some 3,600 troops to 4,500 between now and October.
He welcomed the fact that other Nato nations were placing fewer caveats on what their troops could and could not do in Afghanistan.
He also said he wanted to stress that the UK troops were not seeking to take part in a war on drug production.
They would not be acting as a drugs police, or destroying poppy fields, he said. Instead they hoped to provide the stability which could allow the Afghans to tackle the trade.
"We knew that the Taleban, the drug lords and certain tribal elements would resist any attempt to bring security to the people of Helmand," said Mr Browne.
"Yes, we have taken casualties, but we have overmatched the opposing forces every single time we have faced them. They have tried to block our deployment and failed.
"They will continue to try to disrupt our mission and they will fail again."
Shadow defence secretary Liam Fox said the Conservatives supported the objectives of the UK's mission in Afghanistan but wanted far more detail of how they would be achieved.
He said the price of failure was "intolerable".
Liberal Democrat defence spokesman Nick Harvey welcomed the troop deployment, saying it was "essential that British commanders have all the manpower and equipment they require".
But he said a "clear operational strategy with achievable objectives" was needed.