According to a BBC report on Radio 4's "You and Yours" program, broadcast March 24, an official letter from Westminster City Council states hessian wadding embedded within the ceiling was getting weaker over time and led to the collapse.
According to the BBC, "The local authority is still carrying out an investigation into exactly what happened, but it has sent out a letter to historic theatres in the West End stating that the hessian wadding - a type of sackcloth - mixed into the plaster of Paris, was becoming progressively weak. The health and safety team at the council is recommending that all suspended ornate ceilings are thoroughly inspected as a matter of urgency."
The theatre is due to reopen March 26 with the transfer of the National Theatre of Scotland's co-production of Let the Right One In, seen at the Royal Court last December. The Curious Incident of the Dog in the Night-time will reopen at the adjoining Gielgud Theatre in June.