Labour's republican shadow chancellor is "laid back" over the ceremony required to join the Queen's top advisers and will "do whatever is asked of him", a source close to the left-winger told IBTimes UK.
The clarification comes as speculation mounts over whether McDonnell will kneel before the monarch to join the Privy Council.
McDonnell's office have insisted that there is "no specific ceremonial requirements" they have been informed of by Buckingham Palace, but the shadow chancellor will "adhere to any procedures" asked of him.
Jeremy Corbyn, a close ally of McDonnell, joined the elite group following his first election as Labour leader in 2015.
The Islington MP did not kneel for the Queen, but reportedly kissed the monarch's hand.
The Privy Council draws its membership from the clergy, the judiciary, royalty, the House of Commons and the House of Lords. It holds monthly meetings, typically at Buckingham Palace and Windsor Castle.
Beyond advising the Queen, the councillors are crucially given access to briefings from the security services. In exchange, the establishment figures are expected to make a vow of secrecy in a convention established in 1250 under Henry III.
Downing Street also announced yesterday (29 September) that the Queen has approved of Conservative MPs Sir Oliver Heald and and Brandon Lewis to be sworn into the organisation.