The Prince of Wales and the Duke of Cambridge will open Parliament today after the Queen had to pull out due to mobility issues. They will be acting as Counsellors of State, but what exactly does that mean?
Counsellors of State are individuals empowered by the Sovereign to discharge royal functions under section 6 of the Regency Act 1937. Unlike a Regent, who exercises the full panoply of the Crown’s power, Counsellors of State are meant to handle routine business such as signing state documents or holding meetings of the Privy Council. The Monarch ultimately decides what they can and cannot do, though section 6(1) of the 1937 Act states that the Sovereign can’t delegate the power to dissolve Parliament (except on their express instructions) or create new peerages. Modern convention also dictates that Counsellors of State don’t handle business from the Commonwealth Realms.
When George V appointed the first Counsellors of State in 1911, he selected Prince Arthur of Connaught, the Archbishop of Canterbury, the Lord Chancellor, and the Lord President of the Council. This mix of royalty, ecclesiastics, and politicians remained the norm until the Irish Free State objected to the presence of British politicians among the Counsellors since it meant they were involved in the affairs of the independent nations of the Commonwealth. Since 1930, only members of the Royal Family have served as Counsellors of State. The current law is contained in section 6(2) of the 1937 Act, which provides that the first four people in line for the Throne who are of legal age are eligible to be Counsellors. While someone can be excused if they will be absent from the UK, the Monarch can’t substitute anyone in their place. Traditionally, Counsellors act in pairs, which is why Prince Charles will give the speech alongside his eldest son. Although they will technically be acting jointly, Prince William needn’t do any reading.
While it’s unusual for Counsellors of State to undertake something as high-profile as the State Opening of Parliament, it’s a sensible change under the circumstances. The Queen could have delegated the task to Lords Commissioners like she does for prorogation, but the State Opening is a uniquely important ceremony, and it makes sense for her to entrust the task to senior members of her family. The fact that the Counsellors of State can take over this role is also a testament to the flexibility of Britain’s constitution.
 Sir Edward Ford, one of the Queen’s former Private Secretaries, once characterized the Counsellors of State as “merely a piece of constitutional machinery – the nearest thing to a human rubber stamp that has perhaps yet been devised.” See Vernon Bogdanor, The Monarchy and the Constitution (Oxford: Clarendon Press, 1997), 49.
 For a look at some of the functions that have been delegated to Counsellors of State, see David Torrance, “Regency and Counsellors of State,” House of Commons Library Research Briefing 22 February 2022, 34-35.
 The Regency Act 1937 is a UK statute and as such it doesn’t automatically bind the other Commonwealth Realms. However, some of them have incorporated aspects of it into their own law. For example, section 4 of New Zealand’s Constitution Act 1986 states that “[w]here, under the law of the United Kingdom, the royal functions are being performed in the name and on behalf of the Sovereign by a Regent, the royal functions of the Sovereign in right of New Zealand shall be performed in the name and on behalf of the Sovereign by that Regent.”
 Bogdanor, 47. The convention that Counsellors of State don’t handle Commonwealth matters had not yet solidified.
 The Sovereign’s spouse can also serve, and Queen Elizabeth The Queen Mother was added to the list by the Regency Act 1953.
 With the Duke of Sussex living in America, the number of Counsellors is now effectively three.
 Rodney Brazier has argued that this provision could be tweaked to allow a single Counsellor to act. See “Royal Incapacity and Constitutional Continuity: The Regent and Counsellors of State,” in The Cambridge Law Journal, Vol. 64, No. 2 (Jul., 2005), 379.
 This is analogous to how, when Lords Commissioners deliver speeches on behalf of the Monarch, only the presiding commissioner actually reads anything even though the terms of their Commission states that any three or more of them are supposed to act.