The chairman of the inquiry into the Grenfell Tower disaster has been advised by its chief lawyer to seek assurances any evidence corporate witnesses give will not be used against them in criminal proceedings.

Staff from firms involved in refurbishing the 24-storey block with flammable materials had been due to be cross-examined before they submitted a last-minute legal bid seeking the Attorney General-backed pledge.

Without it, many had threatened to stay silent by claiming the legal right of privilege against self-incrimination.

Counsel to the inquiry Richard Millett QC made the recommendation on Monday by saying if key witnesses refused to answer questions it would “frustrate the exercise of public investigation” and only give the public a “partial picture”.

He told chairman Sir Martin Moore-Bick: “The inquiry needs witnesses to be able to tell the unvarnished truth.”

It comes after the inquiry heard claims the firms were trying to “derail” and “sabotage” the investigation into the fire which killed 72 people in June 2017 by “essentially dictating the terms in which they will provide their assistance”.

Tower block fire in London
Grenfell Tower in west London (Steve Parsons/PA)

Mr Millett said he considered it in the public interest for Sir Martin to ask for the undertaking but expressed “some regret perhaps” over the “timing and manner” in which it was sought.

He told Sir Martin: “Without the full facts found by you based on a complete exploration of the evidence through the witnesses we would like to call, you would not be able to make recommendations at all, let alone recommendations for lasting and deep change.

“Putting it simply, the purpose of criminal proceedings is punishment for the commission of an offence.

“The purpose of this inquiry is to find out exactly what happened at Grenfell Tower, why it happened and who was responsible so that it never happens again.

“That is why you in our submission need to clear the way so that there is absolutely no impediment at all for each and all of the witnesses we seek from going into that witness box and answering our questions on pain of punishment if they don’t.”

Scotland Yard is conducting its own parallel investigation into gross negligence manslaughter, corporate manslaughter and health and safety offences.

Retired appeal court judge Sir Martin is considering the application for firms including external wall subcontractor Harley Facades, main contractor Rydon, architects Studio E and window and cladding fitters Osborne Berry.

Representing victims, Michael Mansfield QC said there is an “overwhelming and strong” consensus opposing the application.

He said: “We say it is abhorrent to the interests of justice that those who are potential perpetrators of this inferno, who have caused the loss of life, injury, which is often overlooked, the loss of homes.

“Then, and continuing now and well into the future, can those potential perpetrators come here and essentially dictate the terms in which they will provide their assistance?”

He said the timing of the application appears to be an “attempt to derail this inquiry”, adding: “If it was serious, this would have been done months ago.”

Stephanie Barwise QC, for another group of victims, said: “In short, the timing of this application is highly disingenuous and bears all the hallmarks of sabotage of this inquiry.

“The firefighters were just as much at risk of prosecution under the Health and Safety at work Act and yet freely gave their evidence without seeking undertakings.”

Fire Brigades Union general secretary Matt Wrack added: “This tactic from corporate witnesses is a complete disgrace, especially at this stage in the inquiry.

“No firefighter sought immunity from prosecution during their evidence to the inquiry. Each and every one of our members did their utmost to give an accurate account of the night.”

Mr Millett urged Sir Martin to come to a “speedy conclusion” in his decision, saying: “This inquiry has proceeded at speed and must continue to do so because there are major questions of public safety which our investigations have revealed and which need to be addressed with the utmost urgency.

“Any delay without a very good reason is not just inconvenient but potentially dangerous.”