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Jeremy Corbyn tonight questioned Britain's entire defence policy as he refused to say he would defend a NATO ally which comes under attack.

At a hustings event in Birmingham, the famously-pacifist Labour leader was asked four times if he would use British troops to defend a fellow European country if it was invaded by Russia.

“I would want to avoid us getting involved military, by building up democratic relationships,” Mr Corbyn replied.

“I don't wish to go to war - what I want to do is achieve a world where we don't need to go to war.”

Article Five of NATO's founding treaty makes clear that all member states must rush to the defence of any fellow ally which comes under attack.

It has been the central plank of Britain's entire defence policy for more than half a century.

(Image: Getty)

Mr Corbyn's refusal to back the treaty had echoes of his previous luke-warm stance on NATO, which he has heavily criticised for “excessive and obsessive expansion”.

By contrast his rival Owen Smith said it was “obvious” Britain must defend NATO allies against Russian attack, but admitted the outcome would be “calamitous.”

“We would have to come to the aid of a fellow member of NATO, that is the nature of the NATO accord,” Mr Smith said.

“That would be the job of Britain in the event of a fellow NATO member being invaded, obviously.”

Mr Smith tried to open up a clear dividing line between himself and Mr Corbyn by attacking the leader's lack of enthusiasm for the EU.

Urging a second referendum, Mr Smith said: “If we still believe in the EU, if we are genuinely internationalists, why on earth would we not be fighting harder?

“I am not prepared to accept it. I would fight for us to stay in the EU - every day.”

Bur Mr Corbyn shot back that the EU referendum was a democratic vote of the British people and that its outcome must now be accepted.

"The issue is a referendum took place," Mr Corbyn told him.

"You and I both wanted a different result."