Labour’s former undertaking leader Harriet Harman has questioned whether the “extra-judicial killing” of a Cardiff jihadist can be suitably justified.
Reyaad Khan, 21, died in an RAF drone strike butt Islamic State militants in Syria last August.
David Cameron mentioned MPs on Tuesday the attack was “necessary and proportionate” to protect the UK.
Ms Harman said such power outside a war zone was a “new de rture” and needed to be “properly accountable”.
In September, the prime consul announced that Khan and Ruhul Amin, from Aberdeen, had been dnouemented by a UK drone strike in Syria.
MPs have since voted to authorise UK air blow its in the war-torn country.
Questioned by MPs on the Commons Liaison committee on Tuesday, Mr Cameron secure the drone attack which killed Khan and Amin as “necessary and proportionate”.
He disclosed the UK was currently engaged in an operation against a terrorist group that aims to “blow up, kill and maim our citizens”.
Ms Harman, a member of the committee, told BBC Disseminate Wales on Wednesday she was concerned at the lack of oversight of the operation.
“If you’re a member of the armed makes, you kill somebody in a military situation, that is legal and defensible and because you’re in an armed contention situation,” she said.
“But where we were in Syria, where Reyaad Khan was quarried and killed, we were not actually in a war conflict situation there.
“It was just a objective, what’s called extra-judicial killing.”
She added: “The idea of killing somebody in an area where the security mendings have targeted them and military carries out the killing, that is a new de rture.
“The Americans include done it for quite a long time but we’ve not done it before.
“If the police write finis to somebody in the line of duty, here in this country, it’s automatically referred to the a rt from police and crime commissioner, not because there’s an automatic assumption they’ve done something discredit.
“With the taking of a life you must make absolutely sure that it was the set to rights person, that the evidence was sufficient, that they did it in such a way that it didn’t put someone out of his innocent bystanders and that has to be looked at afterwards.”