Jeremy Corbyn, a noted republican, will kneel before the Queen and kiss her hand to accept a position on the Privy Council.
The new Labour leader, who has called for the monarchy to be abolished, on Monday accepted the Prime Minister’s invitation to join the ancient council.
It means he will now receive secret briefings on issues of national security.
Friends and aides of the Labour leader had raised doubts about Mr Corbyn agreeing to serve on the Privy Council because of his anti-monarchy views.
While MPs have to swear or affirm allegiance to the Queen, the oath required of Privy Council members is much fuller.
The current wording, which has been adhered to since Tudor times, states: “You will not know or understand of any manner of thing to be attempted, done or spoken against Her Majesty’s Person, Honour, Crown or Dignity Royal, but you will lett [sic] and withstand the same to the uttermost of your power, and either cause it to be revealed to Her Majesty Herself, or to such of Her Privy Council as shall advertise Her Majesty of the same.
David Cameron, who was on a trip to refugee camps in the Middle East, said that he had offered Privy Council membership to the opposition leader on Saturday after congratulating him on his election victory.
The Prime Minister’s spokesman made clear that Mr Corbyn, who is chairman of the Stop The War campaign, will be invited to attend sensitive meetings of the National Security Council. She said: “The Prime Minister has ensured that the leader of the opposition is briefed on national security issues previously, as appropriate, and will continue to look to do that.
Mr Corbyn has in recent days been heavily criticised after he said that he was “unclear” about the point of the drone strike attack which killed a British Isil jihadist who had directed a plot to kill the Queen.
He has also called for the Armed Forces to be abolished.
Mr Corbyn was heavily criticised earlier this month after he said that he could not think of “any circumstances” that would lead him to deploy the Armed Forces.
He also said that Britain was “quite a small country” and questioned whether it could afford its current “global reach”.
Mr Corbyn has also previously said that taxpayers should be allowed to opt out of funding the Army.
In 1996, he challenged the Speaker over whether he could propose a debate about “the future of the monarchy in this country”.
Since being propelled to the Labour leadership, Mr Corbyn has attempted to play down his republicanism, saying that abolition of the monarchy is not a priority.