Firms responsible for the Grenfell Tower refurbishment appeared to know that a planned cladding system would fail if exposed to fire, the public inquiry has heard.
On Tuesday the hearing was told that internal messages from March 2015 show that "the risk which eventuated that night was expressly foreseen by the designers, contractors, and fire safety consultants responsible for the Grenfell Tower refurbishment".
The first phase of the inquiry found the cladding did not comply with building regulations and was the "principal" reason for the rapid and "profoundly shocking" spread of the fire, which killed 72 people.
The second phase of the inquiry into the blaze began on Monday with the inquiry's chief lawyer accusing companies of passing the blame on to each other rather than accept any responsibility.
- Video report by ITV News Correspondent Sejal Karia
Grenfell was built in 1974 but was significantly altered between 2012 and 2016 when combustible aluminium composite material (ACM) cladding was added to the concrete exterior.
The email exchanges were introduced to the inquiry on Tuesday by Craig Orr QC, counsel for Celotex, which supplied the insulation used in the revamp.
They arose as a result of a debate with building control about whether cavity barriers or fire-stops were required, he said.
In an internal Exova email on March 31, a Mr Pearson sent a message to a Mr Ashton, saying: "If significant flames are ejected from the windows, this would lead to failure of the cladding system, with the external surface falling away and exposing the cavity, eliminating the potential for unseen fire spread."
In another message on the same day, a Mr Crawford of Studio E was told by Mr Ashton of Exova that "it is difficult to see how a fire-stop would stay in place in the event of a fire where external flaming occurred as this would cause the zinc cladding to fail".
In his reply Mr Crawford said "this was my point as well - metal cladding always burns and falls off, hence fire stopping is usually just to the back of the cladding line".
Mr Crawford forwarded Mr Ashton's email to a Mr Lawrence of Rydon, and said no-one agreed with building control, to which Mr Lawrence replied: "Excellent. That looks positive," according to a statement submitted to the inquiry.
Several days earlier, an email sent to Harley managing director Ray Bailey, a Daniel Antekell-Jones, also of the company, writes: "There is no point in 'fire stopping', as we all know; the ACM (aluminium composite material) will be gone rather quickly in a fire!"
Mr Orr said: "Whilst expressed in slightly different terms, the substance of what each of Harley, Studio E, Exova and Rydon was openly acknowledging in the above emails, was that the cladding would fail in the event of a fire with external flaming.
"This email exchange is also directly relevant to the claim made by Harley yesterday, that they had no idea and no reason to believe that the primary materials used on the cladding facade would behave as they did on the night of the fire.
"Harley, it appears, knew that the cladding would fail, indeed would fail rather quickly."
He said the "absence of any investigation of the fire-safety risks of Reynobond PE panels" was "all the more striking" given that the firms appeared to know that the panels would fail if exposed to fire with external flaming.
Exova earlier said criticism of it is "unjustified", because it was not consulted about the combustible materials that eventually coated the building.
The firm's counsel, Michael Douglas QC, said it had been "left out" of planning discussions and had been effectively sidelined after Rydon became the main contractor in 2014.
"Exova took no part and was not involved or expected in discussions about materials to be selected for the exterior cladding," he said.
Both the Reynobond PE cladding panels and the Celotex RS5000 insulation were combustible and found to be a key factor in the fire's rapid spread, which reduced the west London high-rise block to a charred wreck.
Grenfell United, a group representing survivors of the fire and relatives of those killed, said they hoped the inquiry would "expose" those who authorised the "devastating refurbishment" of the building's cladding between 2012 and 2016.
"Those responsible continue to deflect blame and we have to suffer their persistent refusal to accept accountability for the preventable loss of life," the group said.
"At the end of this process it will be clear that criminal charges must be brought for the deaths of our loved ones."
A small group of protesters gathered outside the inquiry's venue on Monday morning, chanting: "Justice for Grenfell - we want the truth."
The second phase of the inquiry will examine how the cladding products were tested, and how the residents' complaints to the tower's management were dealt with.
It is due to run until June 2021, and more than 93,000 documents have so far been disclosed, the inquiry said.