A ceiling collapse at London's Apollo 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 was closed on 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 lash 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.
A West End theatre, where 80 people were left injured after its ceiling collapsed in December, is set to re-open next month.
Ten square yards of plaster fell onto the audience below during a performance of The Curious Incident Of The Dog In The Night Time on December 19.
Nimax Theatres, who owns the Grade II listed theatre, said the venue will re-open on March 26.
Nica Burns, chief executive of Nimax Theatres, told the London Evening Standard she hoped to open with a new show.
The cause of the incident is still being investigated but the Metropolitan Police said there was no criminal act involved.
Andrew Lloyd Webber had previously raised concerns about the safety of the theatre, and had suggested it should be knocked down.
The Apollo Theatre's previous owner Andrew Lloyd Webber warned that the building was in a "shocking" state 10 years ago and recommended that it should be demolished, it was reported.
In 2000, Lord Lloyd-Webber, who owned sold the Apollo to Nimax in 2005, told The Times: "The Apollo in particular is a shocking place.
"I suggested that both it and the Lyric should be knocked down and replaced by top-quality modern theatres."
The composer and musical theatre impresario complained that his plans for black-box auditorium inside the existing plasterwork had been opposed by English Heritage.
When it listed the building in 1972, the public body cited the Apollo's "richly ornamental shallow domed ceiling on pendentives" as one of its stand-out features.
One of the lines of inquiry being considered is that excess water during a torrential downpour may have caused the collapse, after forecasters confirmed there was an abnormally high concentration of rain, hail and lightning in London on the night of the collapse.
Initial safety checks at a West End theatre where a ceiling collapsed onto members of the audience have concluded and the venue has been handed back to its owner, Nimax Theatres.
Nimax Theatres said that it is continuing to assist the authorities as they investigate why around 10 square metres of plaster from the Grade II-listed Apollo Theatre plummeted on to the stalls below, leaving 80 wounded.
A company spokeswoman said: "The relevant authorities handed the theatre back to Nimax at 4.30pm today which meant we could commence the process of carefully recovering and logging all personal effects left in the theatre following its evacuation and subsequently return them to their owners."
Westminster City Council added: "The building is safe for our investigators to go in but we cannot confirm if it is safe to open until our investigations are concluded."
A mother has described the moment the ceiling of a West End theatre collapsed, injuring her and her son. Novelist Freya North and 12 year old Felix were among 76 people hurt during a performance at the Apollo Theatre, last night. Three remain in hospital.
ITV News reporter Ben Chapman reports:
The Metropolitan Police has said it is satisfied there was no criminal act involved in the ceiling collapse at the London Apollo theatre and the investigation will be continued by Westminster City Council.
Chief Superintendent Paul Rickett said: "Last night's emergency response demonstrates the true benefits of having tried and tested multi-agency plans. Alongside colleagues from the London Ambulance Service and London Fire Brigade we delivered a response that was swift and effective.
"I would like to thank the public who were caught up in the incident last night for their support that allowed us to get the job done.
"I know that due to the sudden way in which people had to leave the theatre they have left behind items of their property.
"The Apollo Theatre are working hard to ensure that property is collected and safely stored, however due to the damage inside the property and the steps that need to be taken next it may be some days before property can be returned. Thank you for your patience and understanding."
Seventy-nine people were injured after the ceiling collapsed at the Apollo Theatre on Shaftesbury Avenue last night, a London Ambulance Service spokesperson said.
"By the time the scene had been completely cleared, we had treated 79 patients," the spokesperson said.
Of those patients, nine were seriously injured, they added.
Nick Harding from London Fire Brigade said the audience at the Apollo Theatre were walking around in a dazed condition after the ceiling collapsed.
The Kingsland Fire Station manager said, "People were in a lot of shock ... a lot of them had head injuries and other injuries so it was quite a confused scene."
Two patients remain in hospital in a stable condition following the ceiling collapse at London's Apollo Theatre, St Thomas' Hospital said.
Thirty-nine patients, including five children, were treated at the A&E department following the incident.
The remaining 37 were discharged throughout the night.
"The majority of patients had cuts and bruises, and a small number had fractures", the hospital said in a statement.
The associate director of The Curious Incident of the Dog in the Night-time has thanked the actors, crew and front of house staff at the Apollo Theatre for their "heroic efforts" after the ceiling collapsed: