A campaign to keep live performance at a historic theatre has suffered a blow after a conservation body backed plans to convert it into a cinema.

English Heritage has officially supported plans to convert Brighton’s Hippodrome into a cinema, describing the redevelopment as the “final opportunity” to save the theatre.

Campaign organiser David Fisher said he disagreed with the English Heritage assessment and that a business plan showing the viability of a revived live performance venue would be submitted to Brighton and Hove City Council in the very near future.

A decision on the future of the 19th century theatre in Middle Street is set to be made at the local authority’s planning committee next month.

Ahead of that meeting English Heritage has written to the council to say that developers Alaska have shown that theatre use is “unlikely to be feasible” at the venue in the “medium term”. The body also said that developers had shown that converting the cinema back to a theatre at a later date would be “achievable” although “not without cost”.

Samantha Johnson, English Heritage’s inspector of historic buildings and area, writes: “The proposals would cause harm to the significance of the building. However, they are likely to represent a final opportunity to save the Hippodrome and would conserve the aesthetic values associated with [architect Frank] Matcham’s phase of works.”

Mr Fisher said other respected bodies, such as The Theatres Trust, did not agree that its existing interior was unsalvageable. His campaign has gathered almost 10,000 signatures with supporters including leading playwrights Sir Alan Ayckbourn and Sir Arnold Wesker.

He said work was ongoing to draw up a detailed business plan showing that it was viable to retain the listed building as a live venue and that he hoped to work with the developers to make this happen.

He added: “I am not convinced that a cinema will work, it will simply take customers off the Odeon.”

Chris Moore, development manager for Alaska, said: “English Heritage has forensically examined our detailed planning application and recognised the extensive steps we have taken to protect this building and the care which has gone into every detail.

“We are grateful to them for this understanding and pragmatic approach, which will reassure the Brighton public, who may have been misinformed about our approach, and will allow them to support this wonderful restoration project.”