A senior architect from the company that designed the Grenfell Tower refurbishment has apologised to victims of the fire in which 72 people died.
Andrzej Kuszell, a director of Studio E, told the inquiry into the disaster he was "really, really sorry", he wanted to "turn the clock back", and the firm lacked tower block experience.
However, he blamed other firms for giving misleading information and said fire safety rules were "not robust".
It came after protests at the hearing.
The second stage of the inquiry into the tragedy on 14 June 2017 is looking into how the 24-storey tower in west London came to be covered in flammable cladding during its refurbishment between 2012 and 2016.
Studio E was given the task of renovating Grenfell Tower because it was working on a new school and leisure centre nearby.
During nearly five hours of questioning Mr Kuszell told the inquiry: "Hindsight now comes into play - we've lived two-and-a-half years since the tragedy and doubtless absolutely every one of us would wish to turn the clock back."
He also said his company lacked experience in working on tall buildings and that "if we (Studio E) had understood that building regulations were not robust" the tragedy might not have happened.
"It really shouldn't have happened, and I'm really, really sorry for all of you and everybody else who was involved in the project," he said.
"Because I can only say to you from my heart that we really wanted to do the absolute best on this project as we could which is why I didn't enjoy having the project being described as an add-on because in our hearts it wasn't an add-on at all."
Monday's hearing was the first time the inquiry has sat since last week's decision by Attorney General Suella Braverman to guarantee some witnesses will not be prosecuted on the basis of what they say at the inquiry.
The inquiry's chairman Sir Martin Moore-Bick had also backed the request for the guarantee from firms that refurbished the building.
Protesters against the move had briefly delayed the hearing as Mr Kuszell began to give evidence.
Shouts of "it's a disgrace" were heard, and one protester asked the chairman: "Have you sold your soul yet, Sir Martin?"
One man, bereaved by the fire, argued with the protesters saying that he and other victims of the fire wanted to hear what witnesses were to say.
Security staff were called and a senior police officer who leads the police investigation into the fire spoke to three men who were shouting. The hearing resumed about 10 minutes later.
Afterwards, one of the protesters, Jonty Leff, told reporters the decision was "outrageous" and a "whitewash".
"It means the inquiry is defunct and the whole thing has to be shut down and they have to move straight to the prosecution," he said.
Sir Martin has stressed the decision does not mean witnesses have automatic immunity from prosecution.
Police are able to use evidence they gather separately to the inquiry, as well as documents produced during it.
Those documents - between some of the many companies involved in the refurbishment - have now begun to be released to the inquiry.
One email from the Kensington and Chelsea council to the architects Studio E showed that cladding manufacture Arconic (AAP) believed the "current choice of cladding" was "dull and lifeless" offering little visual improvement.