Three British soldiers have been killed in Afghanistan, taking the number of UK troops killed in the troubled Asian country since 2001 to 100.

The Ministry of Defence (MOD) said the soldiers, from 2nd Battalion the Parachute Regiment, died in a suicide bomb attack in the Helmand province on Sunday morning.

"At approximately 1100 hours local time, the soldiers were on a routine foot patrol 1km west of their Forward Operating Base in the Upper Sangin Valley, when their patrol suffered a suicide explosive device," the MOD said in a statement.

"Four soldiers were injured in the attack and were evacuated to the medical facility at Camp Bastion. Sadly one soldier was pronounced dead on arrival and despite the best efforts of the medical team, two of the soldiers died as a result of their wounds.

"A fourth soldier is currently receiving treatment for his wounds and is expected to make a good recovery."

Prime Minister Gordon Brown paid tribute to the "bravery and professionalism" of British troops working in Afghanistan.

In a Downing Street statement, Mr Brown said that UK soldiers were helping to turn "a lawless region" into an "emerging democracy".

Gordon Brown, British Prime Minister: "I want to pay tribute to the courage of all the 100 British troops who have given their lives in Afghanistan in the service of their country."

"My first thoughts and condolences are with the families of these soldiers, who died serving in Afghanistan with such distinction," Mr Brown said.

"I want to pay tribute to the courage of all the 100 British troops who have given their lives in Afghanistan in the service of their country.

"The risks they bear and the sacrifices they make should be in our thoughts, not just today but every day.

"They have paid the ultimate price, but they have achieved something of lasting value - helping turn a lawless region sheltering terrorists into an emerging democracy."

He added: "I do not believe democracy in Afghanistan would have survived without Nato and UN support - and British forces have been on the frontline of that international effort, and have acquitted themselves with great bravery and professionalism."

There are nearly 8,000 UK troops serving in Afghanistan, most of them in the unruly south of the country where they are fighting Taliban-backed militias.

But despite the rising death toll, Mr Brown has insisted that British troops will remain in Afghanistan for the foreseeable future.

"The Prime Minister is convinced that staying in Afghanistan is the right thing to do," Mr Brown's spokesman said.

"We have been making progress. There is a great deal to do. This will take time."