"I vigorously defend our action in Afghanistan and Pakistan because al-Qaeda is today the biggest source of threat to our national security - and to the security of peoples lives in Britain," The Guardian quotes him, as saying at the Lord Mayor's Banquet in London.
"I can report that more has been planned and enacted with greater success in this one year to disable al Qaeda than in any year since the original invasion in 2001," he will add.
Brown will try to offer voters a positive view of the campaign, which has so far cost the lives of 233 British troops.
The latest death came on Sunday, a soldier from The Rifles who was shot dead near Sangin in Helmand province.
Lieutenant Colonel David Wakefield, spokesman for Task Force Helmand, said: "It is my sad duty to inform you that this morning a British soldier from the 3 RIFLES Battle Group was shot and killed while on patrol in Helmand Province.
"He died in the course of his duty. He will be missed by us, his comrades, and we will remember him," he added.
Brown will say that the Western military deployment has put al Qaeda on the back foot, and warn that withdrawing the troop can have dangerous implications.
"Make no mistake, al-Qaida has an extensive recruitment network across Africa the Middle East, western Europe - and in the UK. We know that there are still several hundred foreign fighters based in the FATA area of Pakistan travelling to training camps to learn bomb making and weapons skills," he will say.
A recent poll had revealed that seven out of 10 British want the UK forces out of Afghanistan within a year. (ANI)