A British soldier has been shot dead in Afghanistan, taking the total number killed there this year to 100, BBC reported.
The soldier from 1st Battalion The Royal Anglian Regiment died after an incident in central Helmand Province on Monday. Next of kin have been informed.
The total number of UK troops killed since the start of operations in Afghanistan in October 2001 has now reached 237.
Gordon Brown said every death was "a real and personal tragedy".
"Today we mourn together the 100th British fatality in Afghanistan in 2009," the prime minister said.
"My thoughts, and the entire nation's, are with the families and friends of every one of those brave men who have died this year; indeed, with every one of our service personnel who have lost their lives serving our country in Afghanistan since 2001."
Conservative leader David Cameron said the death marked a "sad milestone" and was "a stark reminder of the daily threat our troops face".
This year has been the bloodiest year yet in Britain's eight-year engagement in Afghanistan.
The vast majority of British casualties have come from improvised explosive devices, the Taliban's weapon of choice.
Over the summer, Operation Panther's Claw claimed 10 lives as British forces drove insurgents out of key territory in Helmand ahead of the Afghan presidential election.
The prime minister has recently announced that 500 more troops will be going to Afghanistan, taking the total UK deployment to more than 10,000.
US President Barack Obama announced he would sending 30,000 extra US troops but stressed America would begin to withdraw its military by 2011.
The majority of foreign troops in Afghanistan are under the command of the Nato-led International Security Assistance Force (Isaf).
More than 40 countries are part of the Nato force, which has a combined strength of about 71,000 troops. The US is by far the largest contributor, followed by the UK. Other major contributors include Germany, France and Canada.
The US has lost 931 military personnel, while the death toll for other countries is 367.
'Hardened our determination'
Chief of the Defence Staff Air Chief Marshal Sir Jock Stirrup said every death was "a sad loss".
"Our people face a difficult and dangerous task in Afghanistan, and 2009 has been a particularly challenging year," he said.
"We remember those who have given their lives, the bereaved families and friends who are left behind, and all those who have been injured."
He said the armed forces were "making a real difference, and are building the basis for enduring success in Afghanistan".
Defence Secretary Bob Ainsworth said the UK presence in Afghanistan was "vital".
"In taking the fight to the insurgents, providing security and hope for the Afghan people and building up their own security forces in ever greater numbers, we will create a stable country that will be able to stand on its own two feet."
Gen Sir David Richards, Chief of the General Staff, said the milestone death had hardened "our determination to succeed".
The 100th soldier to die this year was shot in the Nad-e Ali area of Helmand. He has not yet been named.
Spokesman for Task Force Helmand, Lt Col David Wakefield, said: "It is with great sadness I must confirm that a soldier from the 1st Battalion, The Royal Anglian Regiment was shot and killed in Helmand province this afternoon.
"He was one of us, one of our fellow soldiers, and we will remember him."