The serviceman, from 2nd Battalion The Royal Regiment of Fusiliers, was caught up in an explosion while on a foot patrol near Sangin in Helmand Province yesterday, the Ministry of Defence said.
His family has been informed.
Lieutenant Colonel Nick Richardson, spokesman for Task Force Helmand, said: "The death of any comrade brings much sadness to the Task Force but we take consolation from the fact that these deaths are not in vain.
"Our thoughts and prayers are with the family and friends so affected by this tragic event."
News of the 201st fatality came on the heels of Gordon Brown's pledge to press ahead with the "vital" mission in Afghanistan after the UK death toll hit 200 yesterday.
The Prime Minister said news that the grim milestone had been reached, after a soldier died from injuries sustained in a roadside bomb blast, was "deeply tragic".
But he insisted: "My commitment is clear: we must and will make Britain safer by making Afghanistan more stable.
"We will honour and support those who have been killed or wounded in the field of battle. And we will give those who fight on all the support that they need to succeed in this vital mission."
The 200th casualty, from The 2nd Battalion The Royal Welsh, died yesterday at the Royal College of Defence Medicine, in Selly Oak, West Midlands. He had been on vehicle patrol near Musa Qal'eh in Helmand Province when the explosion happened on Thursday morning.
His next of kin has been informed.
The announcement came just hours after three British troops killed providing security for the Afghan elections were named by the Ministry of Defence.
Two of the soldiers died trying to save the third who had been injured in an earlier blast.
Mr Brown said it was a "day of mourning, and also a day of reflection", adding that Britain was "hugely indebted to those who have made the ultimate sacrifice".
"The best way to honour the memory of those who have died is to see that commitment through," he said.
"Three quarters of terrorist plots against Britain come from the border areas of Afghanistan and Pakistan.
"The work of our troops - providing security, through operations like Panther's Claw, building up the Afghan army and police, and allowing politics and economic development to take root - is vital to preventing al Qaeda once again using Afghanistan as a base for terrorist attacks against Britain and other countries."
Defence secretary Bob Ainsworth said it was a "grim day".
"This is a difficult time but we must all take solace from the fact that, although sometimes slow, we have been making good progress in Afghanistan," he said.
Shadow Defence Secretary Liam Fox added: "There is no more stark reminder of the bravery and sacrifice of British troops in Helmand.
"We are fortunate in this country that we have men and women willing to volunteer to defend the freedoms we are all so lucky to enjoy."
All three soldiers named yesterday were members of a foot patrol that was part of Operation Ghartse Kers 4, intended to provide security for a pre-election Shura, or meeting of elders.
Lance Bombardier Matthew Hatton, 23, of 40th Regiment Royal Artillery, was wounded in the initial attack in Sangin on Thursday.
Captain Mark Hale, 42, and Rifleman Daniel Wild, 19, both from 2nd Battalion The Rifles, were carrying him to a helicopter landing zone when there was a second explosion, killing L/Bdr Hatton and Rfn Wild. Capt Hale died of his injuries later in hospital.
Their identities were released as the Taliban staged a car bomb attack near the British embassy in Kabul.
Four people were killed, but no UK service personnel or embassy staff were hurt in the blast outside the headquarters of the Nato-led International Security Assistance Force (Isaf).
The attack has been seen as a message from the Taliban that it can attack anywhere as Afghans gear up for their second direct presidential election on August 20.
Militants have warned people not to vote and have threatened to attack polling sites.
There have been 10 British fatalities in Afghanistan this month.
In July, 22 UK personnel died as operations were stepped up, the highest monthly total since the mission began in October 2001.
Offering his condolences to loved ones of the two latest British casualties in Afghanistan, ISAF spokesman Brigadier-General Eric Tremblay said: "We are extremely saddened by these losses and I extend my most sincere sympathies to the families, friends and comrades of these two soldiers.
"The continued used of improvised explosive devices by insurgents exemplifies their preference for asymmetrical and indiscriminate attacks, as they cannot withstand direct engagements with Afghan and international forces whose main objective is to protect Afghan civilians."
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