In two official photographs released to mark the Prince of Wales’s milestone, Charles is shown surrounded by his wife Camilla, two sons William and Harry and three grandchildren.
A fourth grandchild is on the way after Prince Harry’s wife Meghan, an American former TV actress, announced her pregnancy following the couple’s Windsor Castle wedding this year.
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Charles is set to attend a tea party later on Wednesday with “inspirational people” who are also turning 70 this year.
In the evening, Queen Elizabeth II is holding a banquet which friends, family and European royals are expected to attend.
There will be gun salutes in London and lawmakers will voice their congratulations for the king-in-waiting in parliament.
The Twitter account for Clarence House, Charles’s official residence, featured flying balloons and a slideshow of pictures from throughout the prince’s life to mark the occasion.
Charles also edited an edition of Country Life to mark his birthday. The prince told the magazine about his fondness for red pheasant crumble pie and “groussaka” — a version of the Greek dish moussaka but with grouse instead of lamb.
He also said he allowed red squirrels, which are being pushed out by American grey squirrels in Britain, to run around his home on the Queen’s Balmoral estate in Scotland.
The prince has spent a lifetime forging his own path during a record wait for the throne, overshadowed by public adulation for his mother.
He has battled a string of public relations headaches and accusations of being cold towards his first wife Diana as well as of interfering in political affairs.
Charles sees himself as a “dissident” working against the prevailing political consensus, according to his former spin doctor Mark Bolland.
The prince has been plagued by low self-esteem but has felt driven to do the right thing, Bolland said.
“The trouble is, there isn’t a job description so you have to rather make it up as you go along,” he said.
Charles has been outspoken on issues close to his heart, notably architecture, the environment, farming, faith and alternative medicine.
But he told a BBC documentary marking his 70th birthday: “The idea, somehow, that I’m going to go on in exactly the same way, if I have to succeed, is complete nonsense.
“I’m not that stupid,” he said.
“You operate within the constitutional parameters.”