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.
On the first day of evidence hearings, Mr Millett introduced five expert reports, including one written by fire safety engineer Dr Barbara Lane.
As part of his address, footage of the tower alight with fiery debris raining down its side was shown, as witnesses in the background sobbed.
London Fire Brigade commissioner Dany Cotton described the site as "alien to anything I had ever seen" in a statement to the inquiry, part of which was read out.
Dr Lane wrote that the "stay put" strategy had "effectively failed" barely half an hour after the fire started, at 1.26am.
The fire spread 19 stories within 12 minutes once it took hold of the external facade, but she claimed the key players involved in the 2016 refurbishment had not ascertained how the new system would behave in a fire.
Dr Lane said: "The building envelope itself was therefore a major hazard on the night of the fire.
"The active and passive fire protection measures within Grenfell Tower were required to mitigate an extraordinary event.
"As a result, the consequences were catastrophic."
The abundance of toxic smoke in the stairwell - the sole escape route - caused a "disproportionately high loss of life", she added.
Another expert, Jose L Torero, blasted the inadequacy of building guidelines and tests which allow for "obvious dangers" to be incorporated into cladding systems routinely.
The professor, based at the University of Maryland, also agreed that the spread of smoke and flames, which "rapidly impeded" occupants' escape, had most likely been caused by occupant and firefighter movement.
Meanwhile, in two further reports, experts diverged from the Metropolitan Police's assessment of the origin of the fire.
Professor Luke Bisby said there was "insufficient evidence" that the fire started by a fridge-freezer, while forensic scientist Professor Niamh Nic Daeid said the origin was "undetermined".
Mr Millett said the inquiry must proceed quickly "in light of the obvious risk to public safety posed by exterior fires on residential tower blocks".
The inquiry is seeking written statements from the corporate and government bodies that are core participants, and Mr Millett warned them to "resist the temptation to indulge in a merry-go-round of buck passing" and identify their role in the chain of events leading to the fire.
Phase two of the inquiry is not estimated to get under way before April or May 2019.