Two British citizens fighting for terrorist group ISIS in Syria were killed in an RAF drone strike on 21 August.
The Register understands it is the first time British nationals have been targeted and killed in a strike by an RAF Reaper unmanned aerial vehicle.
The deceased British passport holders were revealed to be Cardiff-born Reyaad Khan and Ruhul Amin from Aberdeen.
Both were targeted in Raqqa, the Prime Minister revealed to MPs, noting that the strike also killed a third fighter who it may be assumed did not hold British citizenship.
David Cameron told the House of Commons that the legality of the operation was founded upon "the UK's inherent right to self-defence", although the legal advice provided to the government by the Attorney General will not be released.
"There was clear evidence of the individuals in question planning and directing armed attacks against the UK," said the Prime Minister.
"The air strike was the only feasible means of effectively disrupting the attacks planned and directed by this individual," he added, referring to Reyaad Khan.
Cameron, claiming that the police and security services had stopped at least six terrorist attacks against Britain within the last 12 months, declared:
I am not prepared to stand here in the aftermath of a terrorist attack on our streets and have to explain to the House why I did not take the chance to prevent it when I could have done.