Deaths of three soldiers over two days took the British toll in Afghanistan since 2001 to 256, one more than the number killed in the Falklands.
Two soldiers, from the Royal Scots Borderers, 1st Battalion The Royal Regiment of Scotland, were killed by an explosion in Sangin in Helmand Province on Sunday.
"They were on a foot patrol bringing security to local people... when the explosion caught them... Two of our comrades have been cruelly taken from us, but their bravery and fortitude will not be forgotten," said a spokesman.
On Monday a bomb disposal expert, from 36 Engineer Regiment, was killed by a blast as he cleared a path in Helmand.
The news is a new blow to Brown, who recently pledged 500 extra troops and hosted a London conference on Afghanistan, but is struggling to avoid defeat to the main opposition Conservatives in elections due by June.
In contrast premier Margaret Thatcher's popularity surged when she sent an expeditionary force to oust invading Argentinian troops from the Falkland Islands, known in Spanish as the Malvinas, in a 74-day war that left 649 Argentines as well as 255 Britons dead.
Commenting after the news of Sunday's deaths, Defence Secretary Bob Ainsworth said every fatality was "a tragedy" but insisted Britain must stand firm in Afghanistan.
The country is braced for a surge in casualties as a major international assault against Taliban-led militants in the southern province of Helmand is expected to begin within days.
"We have seen an intense, hard and bloody period in Afghanistan but, as we prepare to consolidate our progress with the launch of Operation Mushtarak, it is imperative that we hold our resolve," Ainsworth said.
Afghan Defence Minister Abdul Rahim Wardak added that he was "deeply saddened" by the British fatalities, offering his "sincere condolences" to their families and friends on behalf of the Afghan government.
"I look forward to the day when the Afghan security forces are able to take full responsibility for Afghanistan's security. We are trying very hard to make this day happen as soon as possible," he said in a statement issued by the MoD.
Britain has 9,500 service personnel in Afghanistan and reports suggest up to 4,000 will be involved in Operation Mushtarak.
The Falklands conflict was very different from Afghanistan, a conventional war between two nations rather than soldiers battling insurgents and threatened by roadside bombs or sniper fire.
"The Argentines acted as we did, in an honourable way. Afghanistan is very different. Our boys and girls are fighting people using essentially guerrilla tactics," said Falklands veteran Simon Weston, a former Welsh Guardsman.
But, he added: "Both forms of war bring the same level of despair for the soldier. One is very demoralising over a long period of time, the other is high impact and high intensity."
There are more than 110,000 international troops fighting the Taliban-led insurgents under US and NATO command, with another 40,000 arriving as part of a counter-insurgency strategy designed to bring an end to the war.
In southern Afghanistan Monday, NATO commanders urged the Taliban to surrender -- but Yousuf Ahmadi, a purported Taliban spokesman, told AFP by telephone: "Taliban fighters prefer to stay and fight."
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