Fifteen UK soldiers have lost their lives in Afghanistan in the last 10 days
Foreign Secretary David Miliband has insisted fighting in Afghanistan is key to ensuring UK security, after eight soldiers were killed in 24 hours.
Some 184 service personnel have died there since 2001, more than the 179 killed during the war in Iraq.
But Mr Miliband dismissed calls for UK forces to withdraw, saying they were stopping Afghanistan becoming "a launch pad for attacks" by terrorists.
"This is about the future of Britain," he added.
Fifteen soldiers have died in 10 days in southern Afghanistan as UK troops continue Operation Panchai Palang, or Panther's Claw.
This major assault against the Taliban in Helmand aims to improve security ahead of next month's Afghan elections.
Lt Col Nick Richardson, in Afghanistan, told the BBC it had resulted in more casualties because troops had been "taking the battle to the enemy".
"It's the only way to secure a future for Afghanistan and ultimately eliminate the risk posed to the international community that the Taliban and insurgents there bring," he said.
BBC defence and security correspondent Rob Watson
When British troops were first deployed to southern Afghanistan three years ago the then defence secretary expressed the hope that they would complete their mission without a shot being fired.
It has instead been the most high intensity fighting British troops have faced since the Korean War in the 1950s.
To critics, the ferocity of the fighting is proof of how ill thought out the whole mission has been all along.
Defenders of the operation, however, say it was always bound to be difficult and that the casualties while regrettable have been suffered in a worthwhile and winnable cause.
Certainly the deployment to Afghanistan of around 10% of Britain's army has proved a real strain on manpower, equipment and finances.
For now at least though, Britain remains firmly committed to staying the course.
"We ask people to remember 9/11 and 7/7 and ask themselves whether they thought trying to prevent this from happening would be a worthwhile cause."
The Stop the War coalition has announced an emergency protest in London on Monday, calling for British troops to be withdrawn from Afghanistan in light of the heavy losses.
A group spokesman said: "The troop surge which was meant to pacify Helmand province has become a nightmare for the British army.
"This unwinnable war must stop now."
However, Mr Miliband said troops were there to "ensure that Afghanistan can not again become an incubator for terrorism and a launching pad for attacks on us".
"This is about the future of Britain because we know that the borderlands of Afghanistan and Pakistan have been used to launch terrible attacks, not just on the US but on Britain as well," he added.
Britain's mission in Afghanistan would not be over until the country was secure, he said.
Mr Miliband refuted claims by Conservative leader David Cameron that those fighting on the front line were not properly equipped - particularly with helicopters.
The foreign secretary told BBC Radio 4's Today programme the government had spent £10bn on equipment for force protection - including 1,200 new vehicles - in the last five years.
Former defence secretary John Hutton told the BBC it could be time to "tilt the balance" away from funding high-tech equipment for conflict between nations, towards resources for counter insurgency operations.
And Chancellor Alistair Darling said the government would always ensure that front-line troops were well-resourced.
"You can't send troops into the front line and not be prepared to see it through in terms of the equipment, the resources that they need," he added.
BBC defence correspondent Caroline Wyatt said many on the front line talked about helicopters being in short supply.
But she added: "Some say they have better personal kit than they've had before.
"They do complain about the sheer weight of it. It was 45C there two weeks ago... it's physically absolutely exhausting."
On Friday in Helmand, five soldiers from the 2nd Battalion The Rifles were killed in two separate blasts near Sangin, while a member of the 2nd Royal Tank Regiment died near Nad Ali.
A day earlier, a soldier from 4th Battalion The Rifles was killed in a blast near Nad Ali while another from Princess of Wales's Royal Regiment, attached to 1st Battalion Welsh Guards, was killed fighting insurgent forces near Lashkar Gah.
HAVE YOUR SAY
The British soldiers must suspend all activities in Afghanistan and come home
Col Richard Kemp, who commanded troops in Afghanistan between 2003 and 2004, said: "You develop immense bonds of comradeship between all ranks, and therefore if one of your number is killed or even seriously wounded... it hits you hard."
But he added that the Taliban were suffering casualties "in the region of a hundred enemy dead for every one of our dead", which were not being reported.
Jane Ford, whose son Pte Ben Ford, of Chesterfield, Derbyshire, died in an explosion in Afghanistan in 2007, backed the UK's mission.
"We've got to say to these lads: 'You're doing a good job'," she said.
"We are sorry they're losing their lives, it's awful. But if we pull out now, that bully [the Taliban] has won."
BRITISH CASUALTIES IN AFGHANISTAN MARCH 2006 - JULY 2009
1: Highest monthly toll with 19 dead including 12 killed when a RAF Nimrod crashes in Afghanistan.
2: British death toll reaches 100. Among the 13 fatalities in June is the first British female soldier.
3: British casualties surge as major offensive against Taliban begins in the south. Many are lost to powerful Improvised Explosive Devices.