Whitehall officials have been reviewing access to secret intelligence for the leader of the opposition amid warnings over Jeremy Corbyn’s controversial views on foreign policy.
Should he become Labour leader, Mr Corbyn could expect, by convention, to be invited to attend occasional national security briefings.
Civil servants at the Cabinet Office have now sought expert advice on the protocol for granting access to secret documents to opposition leaders.
Mr Corbyn believes that Britain should leave Nato and supports the abolition of the nuclear deterrent.
Traditionally, the leader of the opposition is invited to become a privy counsellor, granting him or her the right to access to confidential national security information on “privy council terms”.
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However, Mr Corbyn, has suggested that he may not join the Privy Council. “I am quite capable of having private discussions with anybody whether I have got a handle on my name or not,” he said recently.
A Cabinet Office spokeswoman denied that planning was taking place for a Corbyn victory, saying only: “There is a long-standing tradition of the government of the day providing access to intelligence as necessary.”