Hundreds of people lived in the 120 four-bedroom homes in Grenfell Tower, a 1970s-era concrete council building in west London. At least 50 were taken to hospital with injuries or suffering from smoke inhalation.

Firefighters confirmed “a number of fatalities”. But due to the size and complexity of the building – and the chaos that followed the fire as residents fled in all directions – it would take some time to know how many had died.

“The building is full of children, it’s all families,” one resident said.

The tragedy will raise new questions about the safety of the huge tower blocks dotted across London and the UK, and the ‘stay-put principle’ under which firefighters advise residents to stay in their apartments when a fire breaks out.

Eyewitnesses heard screams from residents trapped on upper floors, and saw flashes of lights as they used their phones to signal for help.

Furious residents who escaped the inferno said they had been told to stay in their homes, but had ignored the advice.

Firefighters with breathing apparatus went into the smoke-filled building repeatedly to try to rescue residents.

There were also reports of people jumping from windows to escape the flames, which gutted the building as the fire raged for several hours.

The building appeared to either have no central fire alarm, or it had not been working.

The fire began just before 1am on Wednesday, starting on one of the lower floors and spreading quickly upwards.

Dany Cotton, Commissioner of the London Fire Brigade, called it an "unprecedented incident".

"In 25 years I have never seen anything of this scale," she said.

Over 200 firefighters and 40 fire engines were at the scene.

Crews wearing extended breathing apparatus "have been working in extremely challenging and difficult conditions to rescue people and to bring this major fire under control", she said.

"At this time I am very sad to confirm that there have been a number of fatalities."

Commander Stuart Cundy, from the Metropolitan Police, said police would soon contact next of kin of those killed.

“It is likely to take some time before we are in a position to confirm the cause of the fire,” he said.

There is likely to be an extensive inquest.

The London Ambulance Service said they had taken over 50 patients to five hospitals, with more than 100 medics working at the scene, alongside a “hazardous area response team”.

London mayor Sadiq Khan said a number of people were unaccounted for.

There were big questions that needed answering, on the advice tenants received about staying put, and recent maintenance to the building, he said.

“All of these questions of course deserve a full answer and explanation,” he said.

“We have many many tower blocks of similar scale across London, across the country,” he told LBC radio. “We’ve got to make sure we do everything we can to keep our citizens safe.”

He hoped the tragedy would compel the councils and private management organisations that run such estates to take more fire precautions, if needed.

Kensington council said its “focus for now is supporting rescue and relief operation. The causes of the fire will be fully investigated”.

Australian Jody Martin, who lives nearby, said he grabbed a fire axe and ran into the building to help people escape.

But the smoke quickly became too thick for him to stay inside, he said.

He ran outside the building and screamed at residents to get out.

“They were screaming back to me ‘we can’t get through the smoke, we’re trapped in here’,” he told a radio station.

“I’ve just been watching the apartments of people at the windows slowly screaming at the windows, flashing their phones for help, and slowly each apartment gets burnt out.”

He saw people in smoke-filled apartments “slowly slump down”, he said.

Another witness, Sofia, said she heard many young girls crying out for help.

A building resident told the BBC there had recently been works on the gas supply to the building.

She added that the fire department had come to check the building on the weekend.

"They said it was fine - it's not fine," she said.

She and another resident said there had been no central fire alarm in the building.

Several residents have said that residents had been previously advised to stay in their apartments if there was a fire.

One, Michael, told the BBC they had been told "if ever there was an outbreak of fire you must stay in the premises, they're fireproofed for up to an hour and by then you would have been rescued".

"But if we had stayed in the flat we would have perished," he said.

Michael said he had escaped in his boxer shorts through smoky corridors carrying his daughter wrapped in a dressing gown.

"Everyone was in shock, everyone was fleeing, screaming," he said. "The block was just going up, it was like pyrotechnics, it was just unbelievable how quick it was burning."

Paul Munakr, from the seventh floor, told the BBC he had been alerted to the fire by the sound of people outside shouting “don’t jump”.

"As I was going down the stairs, there were firefighters, truly amazing firefighters that were actually going upstairs, to the fire, trying to get as many people out the building as possible," he said.

Churches, mosques and community centres in the area opened their doors to those fleeing the blaze.

Councillor Robert Atkinson of the Royal Borough of Kensington and Chelsea told the BBC "one of the questions we will be asking is why the fire alarm system didn't work". 

"We had received assurances that the system was looked at and it was given the all-clear," he said. "There are very serious questions to be asked as to how this could have happened."

A recently-completed refurbishment to the building added external cladding and a new communal heating system.

It is managed by Kensington and Chelsea Tenant Management Organisation. A community group called the Grenfell Action Group warned in November last year that “a catastrophic event will expose the ineptitude and incompetence of our landlord” – most likely “a serious fire”.

The tragedy is the worst, but not the first of its kind.

In July 2009 three women and three young children died in a fire in Lakanal House in Camberwell, south London.

An inquest in 2013 found that the fire began with an electrical fault in a television and spread rapidly. The council had failed to carry out proper fire inspection, and work in the 1980s had removed fire-stopping features.

The fire brigade was also criticised for its operaters advising some victims to stay in smoke-filled flats. Those who died had been advised to remain in their flats or did not know about escape routes.

The coroner had recommended that landlords fit high-rise blocks with sprinkler systems.

In 2010 two firefighters died in a tower block fire in Southampton.