The Queen has taken to an open-top vehicle to inspect a ceremonial military unit which owes its existence to her father King George VI.
Standing in a state review Land Rover, the Queen was driven passed the massed ranks of the King's Troop Royal Artillery - formed in October 1947 at the request of the King who wanted a troop of horse artillery to take part in the great ceremonies of state.
The Troop are famed for firing gun salutes on royal anniversaries and state occasions, and for providing a gun carriage and a team of black horses for state and military funerals.
Equipped with 13-pounder field guns dating from the First World War, the soldiers are superb equestrians who, when not performing ceremonial duties, can be deployed around the world on operations.
After inspecting the ceremonial unit, the Queen took her place on a dias in Hyde Park to receive the royal salute of the servicemen and women as their guns and horses thundered past
The King had requested the Troop be formed in the post-war period as the last batteries of horse drawn artillery were being mechanised and in October 1947 he inspected the new unit.
When signing the visitors' book after the ceremony George famously crossed out the unit's old name of Riding and inserted above it King's, so it read King's Troop, Royal Horse Artillery.
The 70th anniversary of the creation of the King's Troop saw the unit's extended military family attend the celebrations in Hyde Park.
Former officers and their men were dressed in suits and wore their medals and bowler hats - what is known as Walking Out Order - and because it was a royal occasion some wore morning suits and top hats, usually seen at Trooping the Colour.
The wives, girlfriends and partners of those taking part wore summer dresses, under raincoats, with hats or fascinators.
Rain threatened to wash out the occasion but after a brief shower the light downpour passed and the guests avoided having to find shelter.
George VI had inspected the Troop at its former barracks at St John's Wood and one serviceman who was part of the parade that day - October 24 1947 - met the Queen.
Percy Austen, 89, was an 18-year-old doing his National Service when the King visited, and he reminisced with the Queen about that day.
He said afterwards: "It was an honour to meet the Queen, absolutely marvellous - I've actually met her before, I received Maundy money from her a few years ago at Oxford."
The pensioner, from Barkham in Berkshire, added: "I said what keeps me fit is I've got two ponies and a horse which I muck out and clean every day - so I'm working all the time."
The Queen, who is a life-long horse owner and breeder, was reunited with her former racehorse Knock Castle which was gifted to the Troop in August.
Warrant Officer 2nd Class Martin Dennis was holding the reins of the animal as the Queen gently stroked its nose.
She joked to a member of her entourage, saying about her granddaughter Zara Phillips: "Zara found him a bit big."
Mr Dennis said later: "I was telling the Queen how surprisingly honest he is and easy to train, she was saying how well he looks and how well he's managed to have stood here."