The sculpture, "Sundial," disappeared from the grounds of the home -- now the site of The Henry Moore Foundation in Much Hadham, England -- between Tuesday afternoon and Wednesday morning, The Daily Telegraph reported Thursday.
Alice Farren-Bradley of the Art Loss Register said more people are registering sculptures kept on display outdoors -- like "Sundial" -- as metal thefts increase.
" 'Sundial' is of such importance to the estate and the general public as a whole, as a piece by a celebrated artist which sadly cannot be recreated," she said. "It would be worth so much more as a piece of art than its base metal value."
Another Moore work, "Reclining Figure," worth $4.6 million, was stolen in 2005. Police later determined the 12-foot high sculpture had been melted down and sold for as little as $2,300, The Daily Telegraph reported.
Moore died in 1986 at the age of 88.