Sundials are at risk of dying out, a Cambridge expert has suggested, as he lamented the "extraordinary challenge" of getting young people interested in the timepieces.

Dr Frank King estimated sundial experts were now down to their last 100 around the globe.

He said few young people were rigorously tackling the maths required to design the timepieces - with British schools "scraping the surface" when it came to algebra, trigonometry and geometry.

The sundial enthusiast, 76, a former computer science lecturer at Cambridge, is one of the leading experts in the field and designed iconic British timepieces including the 22-metre Noon Mark on the London Stock Exchange, which measures 22-metres long, and the Golden Jubilee sundial outside the House of Lords.

But he said there were few left who knew the science behind how sundials work, or even their religious origins.

He told the BBC: “Sundials are old hat. Those few people who are making innovative sundials are making extensive use of computerised tools to design them - that's a good thing.

"To get young people interested in sundials is extraordinarily challenging.’

Dr King, who is also chairman of the British Sundial Society, said the group had 600 members in its heyday, as recently as 1995. Numbers have now more than halved with Italy and USA retaining some of their own member experts.