The first firefighters at Grenfell Tower may not have expected the outside of the building to be flammable, a senior union official has said.
Dave Green, national officer at the Fire Brigades Union, spoke after claims were raised on the BBC's Panorama that the first crews to arrive believed the blaze was contained and extinguished within the flat where it started.
The programme on Monday night quoted sources as alleging it was only as those firefighters left the building that they saw the blaze was still burning on the outside of the 24-storey tower in north Kensington.
Some 79 people are either confirmed as dead or missing presumed dead after the tragedy in the early hours of June 14.
Mr Green said the claim was "speculation" but that 1970s buildings like Grenfell Tower were designed so that each flat was a box which contained fire within itself, with a non-flammable concrete exterior.
There has been speculation that cladding applied to the outside of the building during an £8.6 million renovation project finished in May 2016 may have played a role in the spread of the fire.
Mr Green said: "Clearly it was a hot night and if the (fire) was fairly close to an open window then potentially the flames could have got outside - if there were net curtains, something like that, it could have got up.
"Then the cladding might well have been smouldering.
"As a firefighter you wouldn't have thought to look outside. We would assume that the outside of the building would not be compromised."
Mr Green, who was a firefighter in Nottinghamshire for 20 years, added: "There is no reason why me as a firefighter would think that the outside of the building was compromised in any way."
London Mayor Sadiq Khan wrote to the Prime Minister on Tuesday, setting out what he believed should happen during the public inquiry into the blaze.
This included a recommendation that it be split into two parts, one looking at the tragedy itself and one looking at what happens in the future.
Meanwhile an emotional account of the inside of Grenfell Tower, purporting to come from a heroic firefighter sent into the high-rise building, has appeared online.
The anonymous account on Facebook describes how, after making an ascent in pitch- black smoke, they had to make the agonising choice between rescuing two people discovered on the 19th or 20th floor or go up to rescue five more the couple said were sheltering on the floor above.
In the in-depth account, on the Save The UK Fire Service page, they wrote: "A quick gauge check showed us that the amount of floors we'd climbed had taken its toll, we were getting low on air. There's no way we could make it to the 23rd and back (down) to the bridgehead.
"The couple were shouting and screaming at us through the coughing, trying to tell us there were 5 more people on the floor above!
"Now I had horrible decisions to make and a very short amount of time to make them.
"In what I think would of been less than a minute these are all the things I had going through my head."
The firefighter added: "Can I accept/live with the thought that saving two lives is better than taking the risk to go up and potentially saving no one?"
Eventually the firefighter and their partner decide to come down with the couple, one of whom was unconscious.
A 52-year-old woman was the fifth victim of the Grenfell Tower blaze to be named, after Scotland Yard announced that the death toll had risen to 79 on Monday.
Khadija Khalloufi, who lived in Grenfell Tower, had not been seen since the fire.
Scotland Yard have also formally identified residents Ya-Haddy Sisi Saye, also known as Khadija Saye, 24, Abufars Ibrahim, 39, Anthony Disson, 65, and Mohammad Alhajali, 23, as among the victims.