Writing in the Guardian, legal commentator Joshua Rozenberg
argues that the attack "appears to be within the law". International law, like English law, doesn't require that you have to wait for an aggressor to strike before retaliating so long as the action is proportionate and necessary, he says. Under Article 51 you have to show that an armed attack is occurring or is imminent.
The US used Article 51 to justify the killing of American-born Anwar al-Awlaki, a a radical American Muslim cleric of Yemeni descent who was linked to attacks and plots around the world, in a US drone strike in Yemen in 2011.
Following a failed attempt to blow up an airliner as it flew into Detroit in 2009, US President Barack Obama
authorised the Central Intelligence Agency to kill Awlaki.
His family said he was not a terrorist and
launched a legal challenge to stop the US executing one of its citizens without any judicial process. In a legal opinion published after Awlaki's killing, US Assistant Attorney General David Barron
cited a 2006 Israeli Supreme Court
decision that targeted killings were a legitimate form of self defence.
Phillipe Sands QC, professor of law at University College London, told the BBC's Today programme that the British government's use of the Article 51 line of argument represented a "new direction" for the UK, which had previously treated cases like this as matters for criminal rather than international law. Now, he said, the US "warlike paradigm" had been adopted instead.
"Planning a future attack at some far away place has never been good enough in international law on the use of self-defence - it has to be imminent and on that we need the evidence," he said. He added that the attorney general needed to make a statement to clarify the legality of the situation.
