A series of bronze plaques worth about £500,000 have been stolen in the third raid at the Victoria and Albert museum in as many months.
Thieves had used tools to force open the wooden showcase
The Italian Renaissance items, which depict religious scenes and date from the 15th and 16th centuries, were taken during opening hours on Wednesday.
Mark Jones, director of the V&A, said: "This appears to have been a
well-planned professional theft."
Thieves stole £90,000 worth of items in raids in October and November.
In the latest theft tools had been used to force open the wooden-framed showcase where the plaques were displayed.
Two of the plaquettes are by Riccio, and are worth between £100,000 and
£150,000, the museum said.
Three from the workshop of Donatello and three by Moderno are each worth
between £30,000 and £45,000.
Museum director Mark Jones said: "The V&A is in the middle of a major programme to upgrade security and replace old display cases with new cases throughout the Museum and many galleries have been completed.
"This theft underlines the need to proceed as rapidly as possible.
"We are talking to the Department for Culture, Media and Sport about how the process of upgrading can be accelerated."
In October thieves smashed a cabinet at the museum and stole nine antiques worth £60,000 and dating from the 15th to the 19th centuries.
As a result, the V&A doubled the number of guards on patrol and new security systems were commissioned.
Less than two months later 15 small Meissen figures, worth about £2,000 each, were stolen from a case in the museum's Ceramics Galleries.