A ceiling collapse at a West End theatre which injured audience members and forced its closure was caused by "the deterioration " of century-old cloth and plaster ties holding up timber frames, according to Westminster Council.
The Grade II-listed Apollo theatre will re-open this week for the first time since December 19 when about 10 square yards of plaster plummeted on to the stalls below, injuring 80 people, during a performance of The Curious Incident Of The Dog In The Night-Time.
The ties, made of hessian and plaster of Paris, were used to la sh together timber frames to support the suspended ceiling and had been in place since the theatre opened in 1901.
A spokeswoman for the central London council said "guidance" would be issued to other theatres and historic buildings built with similar materials.
She said: " Our investigation is still ongoing, however our inquiry to date has led us to understand why the ceiling at the Apollo Theatre failed in December. The principal cause was the deterioration over time of wadding ties which supported the ceiling, thought to be in place since its construction in 1901.
"As a result of this finding, we have a responsibility for health and safety reasons to issue guidance to owners of historic buildings, English Heritage, the National Trust and others regarding ongoing maintenance of similar ceilings.
"Theatre-goers can be reassured that, as a precaution, we are working closely with the Society of London Theatre to roll-out this guidance across the theatre community. We are also working with English Heritage with regards to other historic buildings. Although all historic ceilings are in some way unique, our guidance outlines what precautions owners can take to ensure the safety of this ornate plasterwork, including thorough checks of suspended ceilings of a similar construction in order to preserve the unique heritage of our great London theatres and historic buildings.
"It is likely that our investigation will conclude later in the spring."
The council has confirmed the theatre, which is due to open with an adaptation of the vampire movie Let The Right One In, is safe.
Building safety body Structural Safety has raised concerns in recent years about the need for regular checks on the ceilings of performance venues in the UK.
The organisation has reported on previous instances of ceiling problems and pointed to Home Office recommendations in 1955 for cinema safety, prompted by failures in plasterwork.
The director of Structural-Safety, Dr Alastair Soane, said: "Owners of entertainment buildings have to ensure that their structures, including the ceilings, are regularly inspected and maintained."