Here is what we know so far:
How many victims?
The police have said six people were killed at Grenfell Tower in west London and that they expect that number to rise as emergency workers make their way through the building.
The ambulance service said 64 people had been hospitalised, 20 of whom were in a critical condition.
Several hundred people would have been in the block when the fire broke out, local officials said. The London Fire Brigade said it had rescued “large numbers” of people.
London Mayor Sadiq Khan said firefighters were only able to reach the 12th floor of the 24-storey block at the height of the blaze.
He said “a lot of people” were still unaccounted for and urged any survivors to contact the authorities.
More than 200 firefighters were deployed to put out the fire, which was still burning more than 11 hours after the alarm was raised. Some 100 medics tended the casualties.
Fire services said they received the first call at 00:54 am on Wednesday (2354 GMT Tuesday) and were on scene at the Grenfell Tower public housing complex within six minutes.
Witnesses reported the entire building was on fire by around 2:00 am (0100 GMT). Around an hour later the London Fire Brigade also tweeted that the whole building was in flames, starting from the second floor.
Survivors said they saw the flames outside their windows, rising up the exterior of the building, which had recently been covered in cladding during a major refurbishment finished last year.
“It appears that the external cladding has significantly contributed to the spread of fire,” said Angus Law from the BRE Centre for Fire Safety Engineering at the University of Edinburgh.
Where was the fire?
Grenfell Tower is a concrete block built in 1974 in the working-class area of north Kensington in west London.
The £8.7 million (9.9 million euro, $11 million) refurbishment of the block was completed last year and included new windows and heating system.
The area neighbours the wealthy district of Notting Hill.
What were residents’ concerns?
Construction firm Rydon, which completed a refurbishment of the tower in 2016, said the work “met all required building control, fire regulation and health and safety standards”.
David Collins, former chairman of the Grenfell Tower Residents’ Association, said the building’s management had failed to listen to residents’ calls for improvements on fire safety.
“If the same concerns were had in a wealthy part of Kensington and Chelsea they would have got resolved, but here they didn’t get resolved,” Collins told AFP.
“This is a multi-ethnic, multicultural, diverse community that just didn’t get served by the people representing them,” he said.
Local residents had warned a year ago about a potential fire risk caused by rubbish being allowed to accumulate during the refurbishment.
“This matter is of particular concern as there is only one entry and exit to Grenfell Tower during the improvement works,” read a blog post by the Grenfell Action Group.
“The potential for a fire to break out in the communal area on the walkway does not bear thinking about as residents would be trapped in the building with no way out,” it said.
Some residents said the official advice was that people should stay inside in the event of a fire.
Khan said the advice would be looked into.
“We can’t have is a situation (where) people’s safety is put at risk because of bad advice being given or if it is the case, as it’s been alleged, of tower blocks not being properly serviced and maintained,” he told the BBC.
British Prime Minister Theresa May’s new chief of staff Gavin Barwell, a former housing minister, promised a review of building regulations covering fire safety following a fatal 2009 fire in another London local authority apartment block.
The review has not been published.