Anything said by witnesses to the Grenfell Tower inquiry will not be used to prosecute them over the fire, the attorney general has said.
The second phase of the inquiry has been on hold for several weeks, as many witnesses threatened to stay silent without a guarantee.
The request came from lawyers for those involved in refurbishing the block.
It only applies to oral evidence from individuals and not documents and oral evidence from corporations.
The chairman of the inquiry backed the request earlier this month but had needed approval from the attorney general.
Suella Braverman's office said she had concluded the guarantee was needed to "enable the inquiry to continue to hear vital evidence about the circumstances and causes of the fire".
Without it, she concluded that some witnesses would be likely to decline to give evidence, her office added, by claiming the legal right of privilege against self-incrimination.
Survivors' group Grenfell United said it was a "sad day".
The second phase of the inquiry, which began in January, is looking at how the building came to be covered in flammable cladding during its refurbishment between 2012 and 2016.
In a statement, Ms Braverman 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."
Inquiry chairman Sir Martin Moore-Bick said he sought the pledge to allow individual witnesses to provide 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.
Sir Martin said the Metropolitan Police Service 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.
Survivors' group warning
Grenfell United said the need to establish what happened "must not come at the expense of justice and prosecutions".
"The inquiry is about getting to the truth so that lessons are learnt and the government can make changes," the group said in a statement.
"We take part to make sure there will never be another Grenfell and people are safe in their homes.
"For our continued participation, the government must make sure the inquiry process does not undermine prosecutions.
"We expect criminal prosecutions at the end of this and will not settle for anything less.
"If prosecutions are affected by this decision we will hold the government accountable.
"Grenfell was a tragedy but it was not an accident."
Fire Brigades Union general secretary Matt Wrack said he thought firefighters who gave evidence in the inquiry would be "appalled" that the undertaking had been provided.
Mr Wrack said in a statement: "The truth must now come out - and those responsible must be finally held to account."
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.
The inquiry's first phase found the cladding was the principal reason for the rapid and "profoundly shocking" spread of the fire at the 25-storey building.