Jeremy Corbyn has questioned the legality of the drone strike that is believed to have killed Mohammed Emwazi, known as Jihadi John.
The British national, who was recruited by Isis and appeared in a series of propaganda videos beheading Western hostages, was targeted by a US air strike last Thursday but it has not been confirmed whether he is dead.
US officials said they were “99 per cent sure” he was killed by the strike.
David Cameron defended it as an “act of self defence” because of the threat he posed to British citizens.
But Mr Corbyn told ITV News on Monday that if he was Prime Minister he would “only authorise actions that are legal in the terms of international law."
Asked whether he believed the attack on Mr Emwazi was legal, the Labour leader said: "I question that. Surely if somebody is doing something wrong you act legally against them.
"If we are setting ourselves up as the West, as in accordance with the UN, with international law and of our own laws, then I think we have to act in accordance with them.
"I am awaiting an explanation of where the legal basis was for that incident that went on.
"Obviously people planning things to attack others is wrong, but there is a process to go on about that. That is why I am looking for a political process."
Mr Corbyn said he did not support the "shoot-to-kill" policy in the UK in the event of a terror attack, saying such an approach could "often be counter-productive".
Mr Corbyn also urged Mr Cameron not to cut police numbers in the light of Paris attacks, which he said had proven the need for tighter security.
He said: “We need obviously maximum vigilance and security in this country, I hope he’ll be prepared to engage with us as the Labour opposition on restoring police number and not going ahead with the police cuts because community policing is something that is very important within our society.”
Mr Corbyn also made clear that Labour MPs would not be given a free vote on any Government motion to bomb Isis targets in Syria.
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