The King marked the launch of the world's longest coastal walking route with a two-kilometre walk.
King Charles was pictured in brown brogues as he strolled through part of the King Charles III England Coast Path, at Seven Sisters National Nature Reserve, on Thursday.
Natural England is officially launching the path as the 2,700-mile route nears completion.
It is estimated by officials that the path, which has been nearly two decades in the making, would take a year to walk with weekends off.
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Charles, who walked on the path in a group as he acknowledged passers-by, equipped with a walking staff, was also there to mark the opening of England's newest nature reserve.
The Seven Sisters National Nature Reserve has become the 13th part of the King's series of nature reserves, and marks the halfway point to creating or expanding 25 national nature reserves by 2028.
There have been 13 reserves declared so far, including the chalklands of Seven Sisters, East Sussex, which cover an area of 65 square miles.
Share Willmore, 76, a rambler, said she was out walking with her friend and was left "amazed" to see the King.
She said: "Not what we were expecting at all, just out for a nice walk and someone said the King was along there."
The King, when approaching the white cliffs landmark, said they were the ones "I avoided hitting when I was in the Royal Navy".
He also spoke to a group of cyclists, who were riding e-bikes, and joked they were "cheaters".
The path on the Sussex coast passes over the Seven Sisters cliffs, the site of the new 1,500-hectare (3,700-acre) national nature reserve.
The reserve, stretching across the cliffs and the Cuckmere River, sits within the South Downs National Park, which is home to globally rare habitats including chalk grasslands, heath and floodplain meadows.
Plans for what is now known as the King Charles III England Coast Path were first legislated for in 2009.
It was then renamed to mark the King's coronation. The Seven Sisters part of the route stretches from Eastbourne to Littlehampton.
(c) Sky News 2026: King launches world's longest coastal walking route
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