The thieves took 15 to 20 paintings and a substantial portion of the silver collection from the Westfries Museum in Hoorn, the northern Netherlands.

Among the stolen works were 17th century paintings by Jan van Goyen, Jacob Waben, Matthias Withoos, Jan Rietschoof, Jan Linsen and Herman Henstenburg, up to 10 milllion euro in value.

"The heart of our collection is gone, including top artworks of national importance. They were mostly unique pieces which are irreplaceable," the museum said today.

The museum, in Hoorn, said its advanced security system had been checked on Thursday and that it could not explain how it had been possible. Staff discovered the theft this morning when they found smashed display cases, broken doors and empty frames.

Shown is the stolen 1626 painting by Jacob Waben, A boy with a Horse.