Advice to residents to "stay put" during the Grenfell Tower fire may have made "all the difference between life and death", the public inquiry heard, amid a catalogue of fire safety failings in the building.
Tenants were told to stay in their flats during the June 14 blaze, guidance which was undermined by the block's multiple structural flaws that allowed the fire to breach barriers.
Richard Millett QC, counsel to the inquiry for the Grenfell Tower inquiry, said there had been a "catastrophic failure" of the external cladding to resist the spread of fire.
Cavities which should have prevented flames spreading between floors were installed incorrectly, experts said, while poorly performing fire doors "contributed significantly" to the spread of smoke and fire to the lobbies, impeding escape.
Mr Millett told the hearing that 187 occupants, about 64%, had evacuated the tower by the time the "stay put" advice was formally abandoned at 2.47am.
He said: "It may well be that the withdrawal of the formal 'stay put' guidance at that stage was just that - mere formality in light of the number of occupants that had escaped safely before that time.
"On the other hand, it may be that the formal maintenance of that advice until 2.47am made all the difference between life and death."
The rate of evacuations "substantially slowed" from 1.38am, with just 36 people managing to escape after the stay put guidance was abandoned an hour later, he added.
The 999 call made from flat 16
On the first day of evidence hearings, Mr Millett introduced five expert reports, including one written by fire safety engineer Dr Barbara Lane.
She claimed that the key players involved in the 2016 refurbishment had not ascertained how the new cladding system would behave in a fire.
Fire safety engineer Dr Lane made the remarks in an expert report commissioned by the Grenfell Tower inquiry on the fire protection measures within the 25-storey building.
Tests showed the materials forming the cladding system, individually and together, did not comply "with the recommended fire performance" set out in guidance for a building of that height, the report said.
She wrote: "Additionally, I conclude that the entire system could not adequately resist the spread of fire over the walls having regard to height, use and position of the building.
"Specifically, the assembly failed adequately to resist the spread of fire to an extent that supported the required 'Stay Put' strategy for this high-rise residential building.
"There were multiple catastrophic fire-spread routes created by the construction form and construction detailing."
Listen to Julia Hartley-Brewer discussing the report on talkRADIO's breakfast show.
The windows lacked fire resisting cavity barriers and were surrounded by combustible material, meaning there was a "disproportionately high probability" of fire spreading to the cladding, she added.
The report also claimed key players involved in the 2016 refurbishment were not aware of how the new cladding system would behave in a fire.
Dr Lane wrote: "I have found no evidence yet that any member of the design team or the construction ascertained the fire performance of the rainscreen cladding system materials, nor understood how the assembly performed in fire.
"I have found no evidence that Building Control were either informed or understood how the assembly would perform in a fire."