Grenfell survivors have said they will “not settle for anything less” than criminal prosecutions over the fire, after the Attorney General guaranteed that anything said by witnesses to the public inquiry will not be used to prosecute them over the disaster.

Suella Braverman wrote to Grenfell Tower Inquiry chairman Sir Martin Moore-Bick on Wednesday, confirming that she had accepted the request from staff involved in refurbishing the high-rise block with flammable materials.

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

Survivors and victims group Grenfell United said the ruling marked a “sad day” and that “truth at the inquiry must not come at the expense of justice and prosecutions”.

They added: “Grenfell was a tragedy but it was not an accident.

“The people responsible for knowingly encasing our families in a death trap and the people that allowed them to do it must face the full force of the law.

“We expect criminal prosecutions at the end of this and will not settle for anything less.”

Lawyers from firms including the main contractor and architects involved in revamping the 24-storey tower in west London submitted the last-minute bid for the pledge when the inquiry reopened in January, causing proceedings to be delayed.

Ms Braverman’s office said the Attorney General “had concluded that the undertaking is needed to enable the inquiry to continue to hear vital evidence about the circumstances and causes of the fire. Without it she has concluded that some witnesses would be likely to decline to give evidence”.

In a statement, she said: “The undertaking I am providing to the inquiry means it can continue to take evidence from witnesses who otherwise would likely refuse to answer questions.

“These questions are important to finding out the truth about the circumstances of the fire. The undertaking will not jeopardise the police investigation or prospects of a future criminal prosecution.”

Fire Brigades Union general secretary Matt Wrack said firefighters who gave evidence in the inquiry would be “appalled” that the undertaking had been provided.

Mr Wrack said in a statement: “It seems there is one rule for them and another for those doing the bidding of the profiteers who turned Grenfell into a death trap.

“The truth must now come out – and those responsible must be finally held to account.”

Sir Martin said he had sought the pledge to allow individual witnesses to furnish the public hearings with a truthful account without fear for the future, allowing him to make recommendations based on the fullest body of evidence possible.

The proposed undertaking will cover oral evidence from individual witnesses only.

It does not mean companies or individuals cannot be prosecuted.

Evidence given to the inquiry in written statements or documents can be used against them in any future prosecution, the Attorney General’s Office said.

Sir Martin said the Metropolitan Police did not suggest that granting the undertaking would “hamper” their concurrent investigations.

Scotland Yard is carrying out its own investigation into possible crimes ranging from gross negligence manslaughter and corporate manslaughter to health and safety offences over the 2017 fire which killed 72 people.

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

The second stage of the inquiry previously heard that the main designers and contractors involved in the refurbishment appeared to predict that the cladding system would fail in a fire, up to two years before the disaster.

Hearings will resume at 10am on Monday, the inquiry said.