Prince Charles will marry his long-term partner Camilla Parker Bowles on 8 April, Clarence House says.
A civil ceremony, followed by a service of dedication, will be held at Windsor Castle. Mrs Parker Bowles will take the title HRH the Duchess of Cornwall.
When the Prince of Wales, 56, becomes King, 57-year-old Camilla will not be known as Queen Camilla but as the Princess Consort, Clarence House added.
Charles said he and his wife-to-be were "absolutely delighted".
The move will end years of speculation on a relationship which has spanned the decades since they first met in 1971.
The wedding will be a civil ceremony, which will be followed by a service of prayer and dedication in St George's Chapel at which the Archbishop of Canterbury, Dr Rowan Williams, will preside.
"The Duke of Edinburgh and I are very happy that the Prince of Wales and Mrs Parker Bowles are to marry," said the Queen, in a statement issued on her behalf by Buckingham Palace.
Charles was 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.
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