Jeremy Corbyn is expected to attend a Privy Council meeting with the Queen on Wednesday, when he will be sworn in as a member of the ancient ceremonial body.
The Labour leader, a lifelong republican, turned down the opportunity to attend the council's gathering last month, citing prior engagements.
As a new councillor, Mr Corbyn should, in line with tradition, kneel on a footstool and kiss the Queen's hand during the short ceremony at Buckingham Palace.
He will also have to vow not to reveal any confidential information he receives.
Membership of the council is granted to the leader of the Opposition to allow them to receive secret briefings from the security services.
There has been some debate over whether Mr Corbyn will be required to kneel before the monarch. His spokesman was not available for comment.
Whatever happens may stay private as Privy Council meetings take place behind closed doors.
Mr Corbyn, who is known for being uncomfortable with the more formal aspects of his role, was widely scrutinised while attending the Remembrance Sunday commemorations last weekend.
He faced accusations from some media of not bowing deeply enough after laying his wreath while others backed his slight bow as acceptable. He was praised for staying to speak to veterans afterwards.
After he is sworn in, Mr Corbyn will be allowed to use the title Right Honourable, which denotes membership of the council.
He previously attracted wide criticism for not singing the national anthem at the Battle of Britain 75th anniversary commemorations, but he sang the anthem at the Cenotaph on Sunday.
Last month he wore a white tie and tails for a state banquet held in honour of the Chinese president.
The Queen is head of the Privy Council and the body advises her as she carries out duties as head of state.
The council also provides administrative support for the leaders of the Commons and Lords and has responsibility for the affairs of 400 institutions, charities and companies incorporated by royal charter.
It has a judicial role as the court of final appeal for UK overseas territories and crown dependencies and for a number of Commonwealth countries.
The body - the oldest form of legislative assembly still functioning in the UK - dates from the time of the Norman kings when the monarch met in private - hence the description Privy - with a group of trusted counsellors who fulfilled the role the cabinet performs today.
Meetings are usually held once a month at Buckingham Palace, Windsor Castle or, occasionally, Balmoral.
There are around 500 privy counsellors, but not all members are required to attend regular meetings. It is usually the Queen, the Lord President Chris Grayling and three ministers, plus any who are being sworn in. Discussions are held standing up, ensuring they do not last too long.