The King’s 75th birthday will be a working day for the monarch marked by gun salutes across the capital.
Charles will officially launch the Coronation Food Project with the Queen which aims to bridge the gap between food waste and food need.
The royal couple will visit a surplus food distribution centre outside London and meet staff and volunteers to hear about the ways in which food waste can be used for social good.
Among the guests will be around 400 nurses and midwives alongside the Chief Nursing Officer of England Dame Ruth May and Amanda Pritchard, chief executive of NHS England.
Birthday celebrations began early for the King when on Monday he was guest of honour at a party staged in the grounds of his Highgrove home in Gloucestershire.
Charles joined a host of people, from community stalwarts nominated by friends and family who were also turning 75 this year, to representatives from organisations marking the same milestone, from the NHS to members of the Windrush generation.
Among the famous faces at the event were celebrity chef Raymond Blanc, The Repair Shop host Jay Blades and singer Leee John from the 1980s group Imagination.
Charles Philip Arthur George was born on November 14 1948 at Buckingham Palace.
After his birth – the first at the royal residence since 1886 – his father the late Duke of Edinburgh drove to Caxton Hall to register the event.
The King is likely to celebrate his milestone birthday privately with family and friends.
At noon, the King’s Troop Royal Horse Artillery, named by Charles’s grandfather King George VI, will fire a 41-gun salute from London’s Green Park.
An hour later, the Honourable Artillery Company will fire a 62-gun salute at Tower Wharf, Tower of London – an extra 21 for the City of London.