The warning comes ahead of a busy summer for UK tourism and follows incidents of crime at Welsh landmarks, including vandalism at Bronllys Castle in Powys.
Specialist insurer Ecclesiastical found that 92 per cent of heritage organisations in a recent survey had experienced crime between 2023 and 2024.
Laura Carter, customer segment director at Ecclesiastical Insurance, said: “As the UK’s leading insurer of Grade I listed buildings, we’re passionate about protecting Britain’s heritage.
“In recent years we’ve seen thefts from heritage properties increase – causing damage to items and places of real historical significance.”
She said heritage organisations should use a mix of physical and electronic security to protect their sites.
Deputy chief constable Rachel Nolan, national policing lead for heritage and cultural property crime, said: “Stealing from or damaging any historic buildings or cultural sites is abhorrent.
“They are attacks on both our national and local heritage.”