The man, who is believed to be from the Coleraine area, was one of two members of Britain's Special Forces ambushed by the Taliban in the Sangin Valley, northern Helmand province on Tuesday.
Speaking during Question Time in the House of Commons yesterday, the Prime Minister described the men as "two brave and committed soldiers".
He said: "I know the whole House will join with me in sending our condolences and sympathy to the families of the
two British soldiers killed in Afghanistan.
"They were fighting the Taliban. They were brave and committed soldiers. This country can be very proud of the work they were doing."
Tory leader David Cameron echoed the Prime Minister's comments, adding: "Our thoughts and prayers are with their families."
Liberal Democrat leader Sir Menzies Campbell offered his party's condolences.
In a statement the Ministry of Defence said it could confirm with "great sadness" the "deaths of two British soldiers in Afghanistan".
"During a planned operation in the Sangin Valley, northern Helmand province, in the early morning of June 27, a UK patrol came under attack," said a spokeswoman.
"Two soldiers died, and one was seriously wounded, although his injuries are not thought to be life threatening.
"Next-of-kin have now been informed."
The MoD added that the men's families had requested that it did not release their names.
The attack took place during a night-time vehicle patrol in the Sangin Valley.
It is thought a rocket-propelled grenade destroyed a British Army
vehicle and the men were killed in the subsequent fighting.
The deaths brought the toll for British forces since the beginning of the conflict to three. A 29-year-old captain was killed in a clash with rebel fighters earlier this month.