The Queen has never faced a major constitutional crisis comparable to that triggered by the abdication of Edward VIII, but she has had her fair share of difficulties.
His reign spanned many British governments under 15 Prime Ministers and dozens more in the Commonwealth.
She received weekly briefings from the Prime Minister at the time, and dozens of government documents passed through her desk each week for formal approval.
Over the years, she has been able to offer advice – and sometimes caution – to a succession of politicians at home and abroad.
Constitutional expert and former MP Sir Robert Rhodes James told the Royal Society of Arts in 1993 how all prime ministers who served the Queen expressed admiration for her deep political knowledge and insight.
“His political experience, in itself, is far greater than any member of the current government or the opposition,” he said.
“She is the most knowledgeable person on national and international issues and world personalities.
“The cliché that knowledge is power is, like most clichés, true – but it does not include the power to make or break ministries.”
The Queen’s political experience was indeed immense and, at times, she needed it all.
The first major tests both came about because the post-war Conservative Party had no formal rules for electing a leader, forcing it to use its royal prerogative to appoint a prime minister twice.
On Sir Winston Churchill’s retirement in 1955, the role passed to his longtime heir, Sir Anthony Eden, who moved to 10 Downing Street.
When Sir Anthony left in 1957, following the Suez crisis, there were two possible successors, Harold Macmillan and RA Butler.
The Queen consulted Eden, Churchill and two cabinet ministers, Lord Salisbury and Lord Kilmuir, who were ineligible because they were peers.
They took polls in the cabinet and among party whips, and all the combined advice offered to the Queen pointed to Macmillan.
The press, however, had widely expected Butler, acting Prime Minister during Eden’s illness, to succeed and were surprised that he was not called to the palace.
But it was Macmillan’s departure in 1963 that would cause the greatest controversy.
The Queen visited her ailing Prime Minister in hospital and was told that a wide party opinion poll had convinced him that Lord Home should be his successor.
It effectively prevented the Queen from taking wider polls – a failure that was to drag her into a nasty political row when it emerged that Macmillan appeared to have at least partially ‘fixed’ the succession.
The row led the Tories to follow Labour’s lead and establish a selection procedure so that the Queen would never again be placed in such an awkward position.
But sometimes his representatives in the Commonwealth seemed less concerned about the consequences of making deeply unpopular decisions on his behalf.
In 1975, Sir John Kerr, the Governor General of Australia, used the prerogative power bestowed on his office to dismiss Prime Minister Gough Whitlam on the money supply issue.
He was perfectly within his constitutional rights to do so – and the Queen was never consulted – but the issue sparked a row that simmers to this day and could have seen Australia become a republic.
Other constitutional problems have arisen in some of the smaller Commonwealth countries.
Many MPs were infuriated in 1983 by the sudden US invasion of the Caribbean island of Grenada, where the Queen was head of state.
It emerged that no attempt had been made to consult the Queen – a lack of courtesy and breach of protocol which further inflamed a delicate situation.
Then, in 1987, Fiji, following a military coup, became a republic outside the Commonwealth.
This prompted the Queen to warn that anyone seeking to remove her Governor-General would “in effect be reneging on their allegiance and loyalty to the Queen” and to lament that “the end of Fijian allegiance to the Crown should have been achieved without that the people of Fiji have had the opportunity to express their opinion on the proposal”.
In 2019, Prime Minister Boris Johnson embroiled the Queen in a constitutional row during his summer recess amid bitter battles at Westminster over Brexit when he asked her to suspend Parliament for more than a month.
The sovereign was required to hold a Privy Council meeting at Balmoral, her private Scottish estate, where, acting on the advice of Mr Johnson, she approved an order temporarily closing – or prolonging – Parliament for five weeks.
Opposition leaders wrote to the Queen in protest and then-Speaker of the House of Commons John Bercow said the move was a “constitutional contempt” intended to stop Parliament debating Brexit.
Ultimately, the Supreme Court ruled that Mr Johnson’s advice to the Queen to suspend Parliament was unlawful as it had the effect of frustrating Parliament.
The Queen’s devoted attention to her constitutional duties was commented on by many of her prime ministers.
Politicians quickly realized that it was best not to go to Buckingham Palace unless fully prepared.
Harold Wilson once admitted he felt like a schoolboy who hadn’t done his homework when the Queen quoted a document he hadn’t read.
In the documentary Elizabeth R, shot on the occasion of her 40 years on the throne, the queen, who was to remain politically neutral, gave her opinion on the importance of her meetings with her prime ministers.
“They unload or tell me what’s going on or if they have any problems, and sometimes I can also help them in some way,” she said.
“They know I can be impartial and it’s kind of nice to feel like a sponge.
“Once in a while you can put your point of view and maybe you didn’t see it from that angle.”
In his memoirs, James Callaghan recorded how the Queen encouraged him as Foreign Secretary in 1976 to take an initiative, which he already had in mind, to solve the Rhodesian problem.
“Inevitably, the Queen’s opinion was enough to tip the scales, as she is an authority on the Commonwealth and I respect her opinion,” he wrote.
The decision fell through thanks to the intransigence of Rhodesian Prime Minister Ian Smith, but Callaghan added: “I have always thought…the Queen’s initiative on Rhodesia was a perfect illustration of how and when the monarch could intervene effectively to advise and encourage his ministers on his own initiative”. extensive experience and with complete constitutional propriety.
The Queen’s last meeting with a Prime Minister came during Tuesday’s hearing with newly elected Conservative leader Liz Truss, which followed outgoing Prime Minister Boris Johnson’s visit as he tendered his resignation.