A Barbara Hepworth sculpture which is insured for £500,000 has been stolen by suspected scrap metal thieves from Dulwich Park in south London.
The bronze piece, called Two Forms (Divided Circle), was cut from its plinth overnight, Trevor Moore of Dulwich Park Friends said.
The price it could fetch as scrap metal would only be a tiny fraction of its value as a complete work.
Southwark Council is offering a reward for the thieves' arrest and conviction.
Mr Moore said it was thought they broke into the park through a gate off the South Circular.
The piece was designed in 1969 and has been in the park since 1970.
A Southwark Council spokesman said it had been insured for £500,000.
A Hepworth piece from a collection in Bangor, north-west Wales, sold at Sotheby's last year for £445,250, nearly three times its pre-sale estimate.
Peter John, leader of Southwark Council, said: "The theft of this important piece of 20th Century public art from Dulwich Park is devastating.
"The theft of public art and metal is becoming a sickening epidemic.
"I would ask the Met Police and their metal theft task force to investigate this theft as a matter of urgency and would ask anyone with any information about the whereabouts of the sculpture to contact us or the police."
Simon Wallis, director of the Hepworth Wakefield, a gallery which celebrates the work of the renowned sculptor, said: "There's no doubt it is a very significant work from the latter part of her career.
"This piece from 1970 was one of those powerful monumental late bronzes.
"It's an important piece and a very beautiful piece, and beautiful to see it in that outdoor setting.
"That's one of the reasons it's so sad that someone's whipped it for the scrap metal. It will be irreplaceable."
Southwark Council is offering a reward of £1,000 for any information leading to the arrest and conviction of the thieves.
The theft comes a day after Scotland Yard launched its first dedicated unit to tackle the growing problem of metal thefts.
The crime is believed to cost about £700m a year.
Dame Barbara Hepworth, who died in 1975 in a fire at her studio in St Ives, Cornwall, is considered to be one of the UK's most important modern sculptors, with her work displayed in museums and public spaces around the world.