The blaze at the historic former Empire Theatre happened in the early hours of Monday.
Burnley Borough Council and Lancashire Fire and Rescue Service have now confirmed that the fire appears to have been started deliberately and have passed the incident on to the police for further investigation.
A spokesman for Burnley Borough Council said: “We’re appalled to hear about the arson attack at the former Empire Theatre
“The council does not own the building but we have taken all reasonable steps to secure the site. It was a concerted and determined attempt to break into the property through steel plating."
In June, Burnley Council approved plans to create a cafe bar in the entrance foyer to the theatre, dealing a blow to campaigners.
However, in August the National Trust lent its support to the theatre, describing it as ‘a hidden gem’.
Theatres adviser Tom Stickland at the Theatres Trust said: “Heritage crime is a blight on the historic buildings in the UK and unfortunately it is all too common for empty theatres to be attacked in this way.
“The reported arson at Burnley Empire is particularly concerning given recent positive discussions about its future that the Theatres Trust have held with the National Trust and other local stakeholders.”
Mr Stickland said once the extent of the damage has been determined the organisation will work with campaign group the Burnley Empire Theatre Trust to assess the next steps.
He added: “Historic England has noted an increase in arson in heritage buildings in the North West in previous years. We would encourage the council and the building’s owners to take steps to remove access to the building from those seeking to harm it.”
Sophie Gibson, acting chairman of Burnley Empire Theatre Trust, described the news as a “great disappointment”, particularly as the trust had reported to Burnley Council that “individuals were still gaining access to the vacant Grade II listed building.”
The Grade II listed Empire remains on the ‘Theatres at Risk’ register.