Barely half an hour after a fire started on the fourth floor of Grenfell Tower, the safety advice in place for residents was believed to be futile.
Tenants had been told to stay in their flats in the event of a blaze, guidance which should never have been issued given the block's structural flaws, an expert found.
Dr Barbara Lane said the cladding system could not adequately resist the spread of flames, meaning a "stay put" strategy for the building was not justifiable.
She wrote in a report for the public inquiry that the plan had "effectively failed" by 1.26am on June 14, around 32 minutes after the first call to emergency services.
The stairwell - Grenfell Tower's only escape route - was "smoke-logged from 1.40am onwards", Dr Lane found.
The stay put guidance was eventually abandoned at 2.47am.
It was unclear why there was such a gap between the advice being rendered redundant and the fire service changing tack, the report said.
"I am particularly concerned by the delay from 2.06am, when a major incident was declared, to 2.47am," Dr Lane continued.
Her views were echoed by another expert commissioned to deliver findings to the inquiry, Jose Torero.
In his report, the fire is split into four stages - and it is said that the stay put advice was questionable from the second stage.
This stage was from 1.05am until 1.30am and covered the time it took for the fire to reach the top of the building from the compartment of origin.
Mr Torero wrote: "A 'stay put' strategy is not consistent with the characteristics of the second stage.
"To a large extent the building remains tenable and the stairs still retain the characteristics required for them to be a safe egress path."
But even when the advice was dispensed with, it was unclear whether every resident was aware they should have evacuated, Dr Lane's report said.
She went on to recommended that blocks of flats have an automatic or manual means of raising an alarm sounder or providing voice alarm announcements, as currently it is not possible to easily communicate changes in advice.
During the Grenfell fire, she wrote, "it is not clear at this stage how the 'all out' message was communicated to residents who were still in the tower".
This would have been particularly problematic for residents who were less mobile.
There also needs to be "serious and urgent" consideration to changing the current approach in buildings enveloped in similar material to Grenfell, Dr Lane recommended.