Witnesses involved in designing and choosing materials for the Grenfell Tower refurbishment are applying to claim "privilege against self-incrimination" to protect themselves from the prospect of prosecution.
Groans filled the hearing room as chairman Sir Martin Moore-Bick said on Wednesday that he had been invited to ask Attorney General Geoffrey Cox QC for an undertaking that "nothing said by a witness in answers to questions in the inquiry will be used in furtherance of a prosecution against them".
It would allow them have the "complete freedom to tell the truth without any concern for the future", he said.
It comes after the inquiry heard that certain key firms which revamped the high-rise block appeared to know two years before the disaster that a planned cladding system would fail if exposed to fire, according to internal emails disclosed on Tuesday.
Scotland Yard said in September that it had completed 17 interviews under caution in its investigation into the fire regarding gross negligence manslaughter, corporate manslaughter and health and safety offences, and said "the number will continue to increase".
The application was made by counsel for firms including external wall subcontractor Harley Facades, main contractor Rydon, and the TMO (tenant management organisation), he said.
Sir Martin said the development had caused him a "little surprise" as "hitherto there has been the fullest co-operation with the inquiry".