Enfield's Initimate Theatre is due for demolition, a decision by the local council that the Theatres Trust calls "hugely disappointing".
Planning permission was granted last week, though the theatre has been on the trust's risk register since 2019 because the church that owns it wants to turn it into a parish hall and accommodation.
While amended plans for a replacement building will ensure some theatre provision, the trust says that, "in reality, the proposals are not practically workable for live performance".
"The stage is inadequate in both height and floor space, audience sightlines would be poor, there is no supporting infrastructure such as sound and lighting, there are no obvious dressing rooms or storage facilities and the capacity is vastly reduced compared to current levels."
The redevelopment will also leave the north London borough without a community theatre space, "for which there is widespread local support and a clear demand," the trust adds.