PA/GETTY Jeremy Corbyn believes Britain should be a republic without the Queen

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The left wing leader, who once voted to abolish the monarchy, has indicated he may refuse to become a member of the Privy Council, the Queen’s formal body of advisers. However, unless he swallows his pride, Corbyn will not be “privy” to state secrets or issues of national security in times of emergency. David Cameron could even order that another member of the Labour party take his place at such key meetings. Declarations of war and peace treaties are signed “on the orders of the Privy Council” thanks to ancient rules first established by William the Conqueror to link inherited monarchy with elected government. The Privy Council, which requires MP members to swear a second oath of allegiance to the Queen, is also involved in making amendments to European treaties.

Meetings of the National Security Council and Cobra, the Government’s crisis response committee could also be off limits for Corbyn if he refuses to join. And it might prevent him from taking part in Prime Minister’s Questions from the despatch box, relegating him to the backbenches for his weekly showdown with David Cameron. “By not taking Privy Council he may as well wave the white flag,” admitted a senior Labour source. “If you aren’t willing to attend meetings of the highest importance then you’ve got no business leading the party and vying to be the country’s Prime Minister.”

Constitutional expert David Rogers, who explores the subject in his new book, “By Royal Appointment: Tales from the Privy Council”, said: “The official Leader of Her Majesty’s Loyal Opposition has always been sworn to the Privy Council.