Arsonists struck at an arts centre which had been recently restored using a £275,000 National Lottery grant.
It had taken volunteers 15 years to establish The Old Meeting House in Helmsley, west Yorkshire, but the fire reduced the theatre to a smouldering ruin in minutes.
Mike Dawson of loss adjuster Davies described the blaze as one of the worst he has seen in his career.
He said: “So many people have made the theatre a visionary venue including Dame Judy Dench and Ian Carmichael.
“I hope we can get the project running again as soon as possible.”