The chair of the inquiry into the Grenfell Tower fire has asked the attorney general for a pledge to prevent oral evidence given by corporate witnesses from being used to prosecute them in the future.

Sir Martin Moore-Bick wrote to Geoffrey Cox QC requesting a pledge which will stop any evidence given in person by staff involved in the block’s refurbishment from being used against them in criminal proceedings.

Many had threatened to remain silent on the grounds they may incriminate themselves.

On Monday, the inquiry’s chief lawyer, Richard Millett QC, advised Sir Martin to accept the request, saying: “Without it you will not get to the truth.”

However, Michael Mansfield QC, for the victims, previously described the move by key firms as “abhorrent”, saying the “potential perpetrators of this inferno” are trying to “essentially dictate the terms in which they will provide their assistance”. Stephanie Barwise QC, for another group of victims, said the timing is “highly disingenuous and bears all the hallmarks of sabotage of this inquiry”.

Grenfell fire remembered two years on: In pictures Show all 12 1 /12 Grenfell fire remembered two years on: In pictures Grenfell fire remembered two years on: In pictures Campaign group Grenfell United project a message on to the side of a tower block in Newcastle ahead of the second anniversary of the Grenfell fire to highlight the number of blocks that are still covered in flammable cladding, despite the role that it played in the fire PA Grenfell fire remembered two years on: In pictures People release balloons in front of the Grenfell Tower during a vigil to mark the second anniversary of the fire (Peter Summers/Getty Images) Grenfell fire remembered two years on: In pictures Downing Street is illuminated green to mark the second anniversary of the Grenfell fire EPA Grenfell fire remembered two years on: In pictures Kensington Palace is illuminated green to mark the second anniversary of the Grenfell fire Getty Grenfell fire remembered two years on: In pictures Campaign group Grenfell United project a message on to the side of a tower block in Salford ahead of the second anniversary of the Grenfell fire to highlight the number of blocks that are still covered in flammable cladding, despite the role that it played in the fire PA Grenfell fire remembered two years on: In pictures Campaign group Grenfell United project a message on to the side of a tower block in by the Grenfell Tower ahead of the second anniversary of the fire to highlight the number of blocks that are still covered in flammable cladding, despite the role that it played in the fire PA Grenfell fire remembered two years on: In pictures Cards bearing names of victims of the Grenfell fire are attached to a railing nearby to the tower Getty Grenfell fire remembered two years on: In pictures People obersve a memorial during a vigil to mark the second anniversary of the Grenfell fire Getty Grenfell fire remembered two years on: In pictures Campaign group Grenfell United project a message on to the side of a tower block in Salford ahead of the second anniversary of the Grenfell fire to highlight the number of blocks that are still covered in flammable cladding, despite the role that it played in the fire PA Grenfell fire remembered two years on: In pictures The Grenfell Tower is illuminated green to mark the second anniversary of the fire Getty Grenfell fire remembered two years on: In pictures Kensington Palace is illuminated green to mark the second anniversary of the Grenfell fire Getty Grenfell fire remembered two years on: In pictures Downing Street is illuminated green to mark the second anniversary of the Grenfell fire EPA

A ruling published on Thursday made clear the rationale behind the request was to allow witnesses to provide a truthful account to the inquiry without fear of the future.

The ruling said: “Any recommendations we may be able to make will depend for their weight and authority on the evidence that underpins our findings ... it is essential that we be able to explore with the witnesses their states of mind and the reasons for their actions.”

It adds: “We think it likely that for many witnesses the pressure of being asked to admit shortcomings with the risk of criminal proceedings in the background will prove too great.”

Scotland Yard is conducting its own investigation into possible crimes ranging from gross negligence manslaughter and corporate manslaughter to health and safety offences.

The pledge would not act as a blanket providing total immunity from prosecution nor cover statements or documents submitted to the inquiry.

Described by the inquiry panel as “limited in its effect”, it would only prevent authorities from bringing a prosecution against a witness based on their oral testimony or using that evidence as part of a prosecution.

But “it does not prevent the prosecuting authorities from making use of answers given by one witness in furtherance of proceedings against another”, according to the ruling.

The ruling said: “We shall write to the attorney general immediately asking him to grant an undertaking in the following terms: No oral evidence given by a natural or legal person before the Inquiry in Modules 1, 2 and 3 will be used in evidence against that person in any criminal proceedings or for the purpose of deciding whether to bring such proceedings...”

Any undertaking will not cover anyone charged with either conspiring to or giving false evidence to the inquiry.

It adds: “It is for the attorney general, of course, to decide whether it would be appropriate for him to give an undertaking and, if so, in what terms.

“It will be for him to balance the competing demands of the inquiry against the need to avoid prejudicing any future criminal proceedings.”

Witnesses from firms involved in the fitting of flammable materials had been due to be cross-examined, but they submitted a last-minute legal bid seeking the pledge last Tuesday.

In the aftermath of the fire two of the firms involved – Rydon Maintenance and Harley Facades – said they would cooperate fully with the investigation.

The application relates to witnesses from firms including external wall subcontractor Harley Facades, main contractor Rydon, architects Studio E, and window and cladding fitters Osborne Berry.

The firms argued such requests for immunity had occurred in other inquests, such as the Bloody Sunday inquiry and the Ladbroke Grove inquiry.

The second stage of the inquiry has heard the main designers, contractors and fire safety consultants appeared to predict two years before the disaster that a planned cladding system would fail if exposed to fire, according to internal emails disclosed to the hearing.

The inquiry continues.