Stones thrown at Roman museum in Verulamium Park in St Albans

Staff at St Albans Museum turned up on Sunday to find smashed windows at the hypocaust - and it's is believed the culprits used flint rocks to cause the damage.

Glass fragments and debris landed on the mosaic and exposed parts of the ancient underfloor heating.

It has been described as a "mindless act of vandalism" and police in St Albans have launched an investigation.

St Albans Chief Inspector Lynda Coates said: "The hypocaust is a really important part of St Albans’ history and we have a duty to protect it from mindless vandalism so future generations can enjoy it.

"It’s very sad that a person or people have tried to damage it. We will do everything in our power to trace those responsible. If you saw people acting suspiciously by the hypocaust at the weekend or know who may be responsible, please call us on 101."

Councillor Anthony Rowlands, responsible for community, leisure and sport for St Albans City and District Council, said: "This is an appalling crime. It is very sad that someone or some people have targeted one of the District’s most important museum and heritage sites in this way.

"This is an act of mindless vandalism and I would ask anyone who can help the police with their inquiries to do just that."

It is thought the vandals struck between 5.30pm on Saturday when the building was closed and 2.30pm the following day when it was due to be re-opened.

No major damage appears to have been caused to the mosaic or hypocaust, and the building has now been re-opened to the public.

The 1,800-year-old hypocaust has been described as "a marvel of Roman engineering" and "an excellent example of the first indoor heating systems installed in Britain".

The intricate mosaic floor is thought to have been part of the reception rooms of a large town house, built around 200 AD near Watling Street, the major Roman road that ran past the city of Verulamium.