The wadding ties, made of a mix of thick hessian and plaster of paris and used to lash together timber frames to support suspended ceilings have deteriorated since the theatre was built in 1901.
Westminster City Council says the Apollo Theatre is safe to open, following the interim work to erect a protection deck underneath the suspended ceiling.
It says that all historic ceilings are checked every three years, in line with a condition on their Premises Licence, imposed by Westminster City Council.
Westminster Council statement:
"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."
Let the Right One In - an adaptation of John Ajvide Lindqvist’s Swedish horror novel and film, reworked for the stage by BAFTA award winning writer Jack Thorne - opens at the Apollo on March 26th.