A ceiling collapse at London's Apollo Theatre has been put down to weak and old materials, the BBC has learned.
Seventy-six people were injured when part of the roof came down during a performance of The Curious Incident of The Dog In The Night-Time in December.
BBC Radio 4's You and Yours has seen a letter from Westminster City Council saying hessian wadding embedded in the ceiling was getting weaker over time.
The material's deterioration led to the collapse, the council said.
The local authority is still carrying out an investigation but has sent a letter to historic theatres in the West End saying the hessian wadding - a type of sackcloth - mixed into the plaster of Paris had become progressively weak.
The council's health and safety team is recommending that all suspended ornate ceilings are thoroughly inspected as a matter of urgency.
In a statement, the council 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."
The council added that theatregoers "can be reassured" that the guidance was being conveyed to other theatres.
"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 said.
The Apollo, a Grade II listed building, is owned by Nimax Theatres. As well as repairs to the ceiling and balcony, the auditorium, front of house and backstage areas have all been refurbished.
Nigel Barker, English Heritage's planning and conservation director for London, said it was important that historic ceilings were inspected regularly.
"Plaster ceilings have been fixed back or 'keyed' to building structures in different ways over the centuries and a common cause of failure is the weakening or detachment of this 'key'," he said.
"[That] is why ensuring that it is regularly inspected by appropriate professionals to understand how the ceiling is attached in a particular building and the condition of that attachment is the best way to manage any risk."
Before the collapse, repair to stonework and other improvements were being carried out, funded by a restoration levy added to ticket prices.
The Apollo will reopen on Wednesday with a play called Let The Right One In, based on the Swedish novel and cult film by John Ajvide Lindqvist.
The Mayor of London, Boris Johnson, said: "I'm delighted that the Apollo is back with this award-winning production. With over 35,000 people visiting West End productions every night, it's a sector that is a huge earner for our economy."