His Holiness Pope Benedict XVI announced it through the Vatican, to shock and surprise.
A poised Queen Beatrix of the Netherlands revealed it in a video uploaded to YouTube, and King Juan Carlos of Spain confirmed it by signing a parliamentary bill.
Queen Elizabeth, however, has vowed that abdicate is something she will never do.
Abdication is, after all, a dirty word in the corridors of Royal palaces, especially after the earthquake caused when her uncle Edward VIII quit the throne in 1936 to marry the twice-divorced Wallis Simpson.
And there is no indication that that will ever change.
Yet with the Queen now into her 91st year, and with the hard-working Duke of Edinburgh choosing to retire last month at the age of 96, plans are afoot which, if implemented, would see Charles appointed King in all but name.
Palace sources have indicated that the Queen has told her inner circle that, if she is still on the throne at the age of 95, she will ask for a piece of legislation called the Regency Act to come into force - granting her eldest son full power to reign even while she still lives, reports the Daily Mail.
I have spoken to a number of high-ranking courtiers who made it clear that preparations for a transition are moving ahead at pace. They have all confirmed that a Regency with Charles taking the lead is now, at the very least, a real possibility.
One senior former member of the Royal Household said: "Out of the profound respect the Queen holds for the institution of monarchy and its stewardship, Her Majesty would want to make sure that she has done everything she can for her country and her people before she hands over. She is dutiful to her core.
"Her Majesty is mindful of her age and wants to make sure when the time comes, the transition of the Crown is seamless.
"I understand the Queen has given the matter considerable thought and believes that, if she is still alive at 95, she will seriously consider passing the reign to Charles."
High-ranking courtiers have all confirmed that a Regency with Charles taking the lead is now, at the very least, a real possibility. Photo / Getty Images
Clarence House - the household of Prince Charles - is making no comment about Plan Regency, as it has been called. However, it is understood to be a matter of increasingly open discussion at court.
But Palace staff responsible for communications have been ordered to be 'up to speed' on the 1937 Regency Act, which grants power to the heir apparent "in the event of the incapacity of the Sovereign through illness, and for the performance of certain of the Royal functions in the name and on behalf of the Sovereign in certain other events".
The development is all the more significant as it comes against the backdrop of momentous shifts at court - a series of senior staff changes and departures at Buckingham Palace and Kensington Palace, home to the young Royals.
The Queen's Private Secretary, Sir Christopher Geidt, 55, resigned his position two weeks ago, sparking turmoil and a 'wholesale regime change' - as revealed in The Mail on Sunday.
His shrewd deputy - former banker, political adviser and television executive Edward Young, 50 - will replace Sir Christopher after a suitable handover period.
I understand that a number of other high-profile figures - many loyal to Sir Christopher - are poised to leave the Royal Households at Buckingham Palace and Kensington Palace after his unexpected exit.
They include Assistant Private Secretary Samantha Cohen, the capable former Press Secretary and Sir Christopher's number three in the private office. Ms Cohen, who is Australian, is said to be a firm favourite with the Queen.
Falklands and Afghanistan veteran, Vice Admiral Tony Johnstone-Burt, the Master of the Queen's Household, is also said to be weighing up his position. Sir Alan Reid, the Keeper of the Privy Purse, had been due to retire at the end of this year but decided to depart when Prince Philip stepped back from public duties, leaving the job to his deputy, Sir Michael Stevens.
At Kensington Palace, the Duchess of Cambridge has hired a new Private Secretary, Catherine Quinn, who is head of the University of Oxford's Said Business School. It is understood she will take up her new role in October.
Prince William's Private Secretary, former Ministry of Defence communications expert Miguel Head, and Prince Harry's Private Secretary, former Blues and Royals Captain Ed Lane Fox, 40, are also said to be considering their options.
Perhaps significantly, there have been no major staff changes at Clarence House, home to Prince Charles.
Prince Charles of Wales could become King in all but name. Photo / Getty Images
Of course, the transition to a world in which significantly more Royal duties are undertaken by the younger generations is already well under way. Next month, the Duke and Duchess of Cambridge will step into their new roles as full-time working Royals, and Prince Harry will be expected to carry out more engagements at his grandmother's side.
"As ever, the Queen wants there to be the minimum of fuss," said the former member of the Royal Household. "Of course, for obvious reasons, abdication is not even a consideration."
That is not in doubt. On her 21st birthday in 1947 the then Princess Elizabeth took to the airwaves to make one of her earliest radio broadcasts. Speaking from Cape Town, where she was on tour with her parents and younger sister, it was as unequivocal as it was momentous.
"I can make my solemn act of dedication with a whole Empire listening," she said. "I should like to make that dedication now. I declare before you all that my whole life, whether it be long or short, shall be devoted to your service and the service of our great imperial family to which we all belong."
It was a conviction that she confirmed in her Coronation Oath on June 2, 1953.
While it may be acceptable for Queen Beatrix and King Juan Carlos to pass the baton to the next generation, Palace aides still believe the 'A word' is not part of Her Majesty's vocabulary.
There is no escaping the passage of time, however, or the fact that the Queen, at 91, is the oldest and longest-serving monarch in the history of these islands - one who is now without the company of Prince Philip, her consort, on the hundreds of engagements she carries out each year.
The monarch is in remarkable health, yet courtiers are increasingly aware that there is only so much a nonagenarian, albeit one with incredible stamina, can realistically be expected to achieve.
Charles has already been filling in for his mother, representing the head of state on foreign trips during her Diamond Jubilee year to Australia and New Zealand.
Earlier this year he went in her place to Canada to mark the nation's 150th anniversary. On a tour of the Persian Gulf, he represented the Crown and was treated like a king with other Royals bowing to him.
More will no doubt come as the Queen gradually scales back her demanding list of duties.
A Regency, however, would ensure a more formal handover - and, indeed, the Act was considered in more recent times when the Queen first acceded to the throne.
Documents show that following the Coronation of 1953, the young Elizabeth II intervened to ensure that Prince Philip - and not her sister Princess Margaret - would take over as Regent if she were to die or succumb to illness or mental incapacity before Charles, then aged four, reached adulthood. This was not, of course, required.
Times have changed, so with the heir apparent fast approaching his 70th birthday next year, what would it mean in practice today?
Certainly, it is not be a decision to be taken lightly.
To safeguard against a coup, a Regent can only be installed once a decision is taken by at least three of the following: the Sovereign's consort, in this case the Duke of Edinburgh; the Lord Chancellor, David Lidington; the Speaker of the House of Commons, John Bercow; the Lord Chief Justice, the incoming Sir Ian Burnett; and Master of the Rolls, Sir Terence Etherton.
Clarence House and Buckingham Palace officials have played down this speculation, as one would expect. But as one insider told me: "We are in uncharted territory right now - a monarchy in transition with a clear shift in power."
SCANDAL OF THE LAST STAND-IN
The last Prince Regent was appointed in 1811 after George III suffered episodes of mental illness and his eldest son was asked to take over his duties.
Affectionately known as Prinny, the future George IV, was notorious for his hedonism - drinking, eating to excess and keeping several mistresses at once.
