Theresa May has agreed to appoint a panel to help oversee the inquiry into the Grenfell Tower disaster following pressure from victims' families.
Two people will "support" chairman Judge Sir Martin Moore-Bick for phase two of his investigation, which is due to start hearing formal evidence later this month.
Survivors' groups had said the inquiry risked being a whitewash unless Theresa May installed a diverse panel to oversee proceedings.
And 156,000 signed a petition calling for individuals from a wide range of backgrounds to sit alongside Sir Martin.
Mrs May made the announcement in a written statement to MPs on Friday, ahead of Monday's Commons debate on the fire, which ripped through the west London tower block last June, killing 71 people.
Grenfell United spokesman Adel Chaoui, who lost four relatives in the fire, and co-led the petition, said survivors were "relieved", but should not have had to campaign for it.
He said: "This panel means that the inquiry will have the expertise it needs to get to the truth of why our community was not listened to when we raised concerns, why such a dangerous refurbishment could be allowed to go ahead and all the issues that led up to that awful night.
"These are the questions that need answered to make sure this never happens again."
The announcement came the day after Mrs May spoke with a group of people affected by the tragedy at Number 10.
Among them was artist Damel Carayol, who lost his niece in the fire and presented the PM with a print of his painting "Grenfell Tower: Eyesore!! Final Straw".
Sir Martin is heading the investigation into the blaze, which killed 71 people, supported by a legal team, civil servants and three assessors to advise on certain matters.
In December, Mrs May said she had decided not to appoint extra panel members, but gave herself the ability to review the decision.
In Friday's statement she said phase two of the inquiry would be the "largest phase in terms of the number of issues to be considered".
She added: "To ensure that the inquiry panel itself also has the necessary breadth of skills and diversity of expertise relevant to the broad range of issues to be considered in phase two, and to best serve the increasing scale and complexity of the inquiry, I have decided to appoint an additional two panel members to support Sir Martin's chairmanship for phase two of the inquiry's work onwards."
She added that she had written to Sir Martin to tell him of the change and would do so again "once suitable panel members have been identified".