The Queen is to miss the state opening of parliament for only the third time in her reign.

The 96-year-old monarch, who is experiencing mobility issues, will not read the Queen’s speech on Tuesday, Buckingham Palace said. The Prince of Wales will read it on her behalf.

As Charles takes on the head of state’s major constitutional duty for the first time, the move – believed to be unprecedented in modern history – will be interpreted as a significant shift in his responsibilities as a king in waiting.

The Queen’s speech will be read by the prince after a new letters patent, authorised by the Queen, was issued that delegated the royal function of opening a new session of parliament to the counsellors of state.

The Duke of Cambridge will also attend the state opening, as a counsellor of state.

As opening parliament is a core constitutional responsibility of the monarch, the letters patent had to be issued to delegate that responsibility to two counsellors of state. There are currently four counsellors of state: Charles, William, Andrew and Harry.

No other functions have been delegated by the Queen. The Duchess of Cornwall will also attend, although she is not a counsellor of state.

The new arrangement will leave the main throne empty during the ceremony, as Charles and Camilla take their usual seats, with William on the opposite side to Camilla. The imperial state crown will still travel to parliament.

In a statement, Buckingham Palace said: “The Queen continues to experience episodic mobility problems, and in consultation with her doctors has reluctantly decided that she will not attend the state opening of parliament tomorrow.”

A No 10 spokesperson said: “The prime minister fully respects the wishes of Her Majesty and is grateful to the Prince of Wales for agreeing to deliver the speech on her behalf.”

The letters patent was issued to cover the state opening. In this instance, it enables Charles and William to jointly exercise that function. No other functions have been delegated by the Queen. The decision was taken on Monday.

The mobility problems are said to be a continuation of those the Queen has experienced since the autumn.

In order to prevent delay or difficulty in the dispatch of public business, the sovereign may issue a letter patent delegating specified “royal functions” to counsellors of state, as provided for in section 6 of the Regency Act 1937.

Last week it was announced the Queen would not be attending this year’s royal garden parties.

The Queen is understood to have a busy diary at Windsor this week with a call with Australia on Monday, and a planned virtual privy council and phone audience with the prime minister on Wednesday. She is expected to undertake some private engagements later in the week.

The Queen has opened every session of parliament since her accession in 1952, except in 1959 and 1963 when she was pregnant with Prince Andrew and Prince Edward respectively.

On those two occasions, the sessions were opened by the lords commissioners, while the lord chancellor read the Queen’s speech.

Last week it was announced that the Queen would not attend any of this summer’s garden parties at Buckingham Palace and at Holyroodhouse in Edinburgh.

Constitutional expert Dr Bob Morris, of UCL’s constitution unit, told the PA news agency that the arrangements, with both Charles and William attending and the prince reading the speech, were unprecedented.

“They’ve gone down the counsellor of state route – that’s one way round it. The Prince of Wales is giving the speech and William is there as his stay and support, as it were,” he said. “It’s a rather odd confection, but it works.”

He added: “It is unprecedented and it’s the way in which the constitution flexes to accommodate unusual circumstances. I can’t think of any possible earlier version of this.”