One of the first firefighters into Grenfell Tower believed the basis of stay-put advice to residents "failed" barely half an hour after the fire began, an inquiry has heard.
Thomas Abell was part of the North Kensington crew that was called to reports of a fourth-floor kitchen fire at 12.54am on June 14 2017.
Residents were told to remain in their flats for almost two hours as an inferno engulfed the building - advice which is feared to have been fatal.
Incident commander Michael Dowden previously told the inquiry into the blaze that he did not believe flames had spread internally after shooting up 19 floors in 12 minutes.
But on Monday, Mr Abell said he found a flat on the fifth floor "fully involved" in the fire at around 1.28am, believing at this point compartmentation had failed.
Compartmentation is a fire safety feature in high-rise blocks that should stop flames spreading beyond the flat of origin.
It is the basis of the stay-put advice.
Mr Abell, who joined the London Fire Brigade in September 2011, told a hearing at Holborn Bars: "In that situation, the floor above the initial fire floor, you would maybe expect there to be a bit of smoke in there.
"The sort of smoke that came out, the thick, black smoke to floor level, would indicate this is a fully involved fire in this flat, which is unusual."
He added visibility was "zero".
The firefighter did not pass the information back to the incident commander via the operational bridgehead on the second floor as he did not have a radio in his breathing apparatus.
He said he was unaware whether his colleague, firefighter Wayne Archer - who had radio capabilities - managed to update anyone.
It is feared that Mr Dowden did not receive crucial information about fire spread internally during the opening stages of the disaster.
Mr Abell continued: "I thought maybe there would be smoke that had got in through the windows, maybe there was a little bit of fire - I didn't expect it would be a fire in there."
Asked what he thought this meant by lead counsel Richard Millett QC, he said: "Compartmentation failed at that point, yes.
"That was significant to me because my job then was try to nip this fire in the bud and put the fire in this compartment out before it spread to the sixth floor."
He was unable to warn anyone else on the fifth floor because his partner had left him alone to attend to the hose and his legs felt "like jelly".
Seventy-two people died as a result of the inferno.
In a written statement, published by the inquiry on Monday, Mr Abell said he "started to feel concerned" by what he had seen.
He wrote: "At this point I was only aware of fire on the fourth floor and fire within a tower block should not spread in this way, however it was obvious to me that this was a fire compartment due to the smoke and poor visibility."
"I remember thinking, 'this is going up like rocket fuel!'," his statement added.
During a dramatic night, Mr Abell was also required to rescue two men from a fifth-floor window using a ladder.
He then noticed a resident on a "very high floor" about to mount an escape using tethered bedsheets, according to the statement.
He wrote: "I was concerned for this male's safety, and I also did not want other residents to follow this male's example of trying to 'rappel' his way down the side of the building. At one point this male was on the window sill of his apartment.
"I shouted as loudly as I could, I instructed him to stay where he was and not to try and exit the building via the window ... I have subsequently met this man after the fire, and now know his name to be Oluwaseun."
Survivor Oluwaseun Talabi has previously described in TV interviews how he planned to shimmy down the building with his daughter on his back.