Prince Charles is set to marry his long-term partner Camilla Parker Bowles, Clarence House says.
No wedding date has been confirmed at this stage, but reports have suggested 6 April is one possible date.
The move will end years of speculation on a relationship which has spanned the decades since they first met in 1971.
The Prince of Wales, who is the heir to the throne, was previously married to Diana, Princess of Wales, who died in a car crash in Paris in 1997.
The princess famously referred to Mrs Parker Bowles as one of the contributing factors in the breakdown of her marriage to Charles.
The couple, who had two sons - princes William and Harry - had divorced when Diana died in Paris, eight years ago.
Conservative leader Michael Howard said he was "delighted" at the news, but Downing Street has refused to comment until a full statement is released by Clarence House.
Mrs Parker Bowles' current position has meant that her status was dogged with problems, including her seating at social functions away from her partner.
Last June, Mrs Parker Bowles was mentioned in the Prince's accounts, which marked a new realm of acceptance.
The marriage is likely to be a sensitive issue because Mrs Parker Bowles is divorced and her former husband is still alive.
If he became king, Charles would be the supreme governor of the Church of England and some Anglicans remain opposed to the remarriage of divorcees.
The church is officially neutral on the issue, but former Archbishop of Canterbury George Carey recently urged the couple to marry.
BBC Royal Correspondent Nicholas Witchell said: "This is a step not without considerable risk by the Royal Family.
"They will be watching very carefully to see how public opinion unfolds."
He said the prospect of Mrs Parker Bowles taking on a subsidiary title like Duchess of Cornwall, but not becoming Princess of Wales and then Queen was "not as easy as it sounds".
"There would have to be primary legislation to achieve such a result as that. Not only in this country but in all the other countries in which the Prince of Wales will supposedly be king," he added.
Last year, a poll indicated that more Britons support Prince Charles marrying Camilla Parker Bowles than oppose it.
Of those who responded to a Populus poll, 32% said they would support Charles if he remarried, while 29% were opposed.
However, most people - 38% - said they did not care, while 2% had no opinion.
Mrs Parker Bowles has joined the Prince at numerous engagements in recent years - mostly at evening events for The Prince's Trust.
Clarence House staff were at pains to point out that she attended these events in a private capacity.
But the impending wedding will now allow her to be at the Prince's side full-time in an official capacity.