Firefighters had put out the initial fridge fire at Grenfell Tower and were leaving the building when the blaze suddenly flared up, it has emerged.

Crews believed they had put out the fire at the London high-rise and were astonished to see flames rising up the side of the building, new reports have claimed.

Shortly after dealing with the fridge fire early last Wednesday, firefighters were telling residents that it was out, BBC Panorama reported.

But, soon after, the 24-storey building was consumed by flames in one of Britain's biggest ever tower block fires that left at least 79 people dead.

Panorama on Monday night reported that firefighters had seen flames "shooting up" the side of the building. Those reports will add weight to claims that it was the cladding on the exterior of Grenfell Tower that caused the fire to spread so rapidly.

View photos Flames tear through Grenfell Tower as smoke billows into the sky Credit: NATALIE OXFORD/AFP More

But Dave Green, national officer at the Fire Brigades Union, said the first firefighters at the scene might not have expected the outside of the building to be flammable.

The union official said that 1970s buildings like Grenfell Tower were designed so each flat was a box that contained fire within itself, with a non-flammable concrete exterior.

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."

View photos Flames tear through Grenfell Tower early last Wednesday Credit: DANIEL LEAL-OLIVAS/AFP More

The programme also disclosed that leaked letters show how ministers were repeatedly warned that fire regulations were not keeping people safe.

It said letters show ministers were warned that people living in high rise blocks such as Grenfell Tower were "at risk".

The dozen letters, sent by the All-Party Parliamentary Fire Safety and Rescue Group in the aftermath of a 2009 fatal fire in Lakanal House, south London, warned the Government "could not afford to wait for another tragedy", according to Panorama.

The Parliamentary group wrote in March 2014: "Surely ... when you already have credible evidence to justify updating ... the guidance ... which will lead to saving of lives, you don't need to wait another three years in addition to the two already spent since the research findings were updated, in order to take action?

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