That rule has come in very handy for Mr Richards, who will have a photograph on permanent display in the London Museum.

Mr Richards took a colour slide of the rally before the first Aldermaston March in 1958, and the slide has been acquired by the London Museum.

The Aldermaston Marches were anti-nuclear weapons rallies held every Easter from 1958 until the mid-1960s.

After meeting in Trafalgar Square, the marchers marched to the Atomic Weapons Research Establishment at Aldermaston, about 80 kilometres away.

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Overall, tens of thousands of marchers took part.

The London Museum's curator of photographs, Jilke Golbach, said the museum was thrilled to have the photograph in its collection.

"While our collection holds a substantial number of photographs depicting political gatherings and protests in the 1950s and 1960s, they are, without exception, black-and-white," she said.

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"It is unusual to see a colour photograph of this event, and this striking image - taken by a then 19-year-old Australian backpacker - will undoutedbly have an impact on contemporary museum visitors."

The Museum of London is located on the Rotunda in Barbican, London, just north of the famous St. Paul's Cathedral.