The Victoria and Albert Museum in London has been struck by its third theft in as many months. Daylight raiders snatched eight small Italian Renaissance bronze plaques worth nearly £500,000.
The theft took place on Wednesday afternoon when tools were used to force open one of the museum's older wooden-framed showcases in a gallery next to the central garden.
Mark Jones, director of the V&A, said: "This appears to have been a well-planned professional theft. The V&A is in the middle of a major programme to upgrade security and replace old display cases throughout the museum and many galleries have been completed. This theft underlines the need to proceed as rapidly as possible."
The museum was talking to the Department for Culture, Media and Sport about how the expensive process could be accelerated, Mr Jones added.
All the plaques are about 10cm high, date from the 15th and 16th centuries and depict religious scenes. The two most valuable were by the Paduan master Riccio, and were valued at about £100,000 and £150,000 each. Three others were by the Italian sculptor Donatello and the final three by Galeazzo Mondella, known as Moderno.
The theft of a group of nine small Chinese jade objects in October and 15 Meissen figures in November, both from the ceramics galleries, had already prompted a security review. The ceramics galleries are closed while new security systems are tested.
The V&A has not been alone in being targeted; the Science Museum and British Museum have also fallen victim in recent months.
The British Museum lost of 15 items, including hairpins, nail-guards and jewellery, from its Oriental gallery, apparently while the museum was open late into the evening.
A rare 16th-century crystal ball that once belonged to John Dee, an adviser to Elizabeth I, and papers relating to his work were stolen from the Science Museum.
Its items were, however, recovered. A man has been has been charged with theft.
The thefts present a dilemma to the museums. If they publicise the losses, it may assist in retrieving the stolen items. But there is a danger of further copycat raids as thieves become aware of potential shortfalls in security.