More than 100 firefighters are tackling a blaze in a top-floor flat in Woolwich in east London.
Flats below the 17th-floor apartment in Dunedin House, Manwood Street, were evacuated, while people nearby were ordered to stay away from the huge plumes of smoke.
There are no reported injuries and the cause of the original fire is unknown.
Footage from the scene show smoke rising from the top floor of the building.
Firefighters are simultaneously tackling a blaze involving around one hectare of grass opposite the block, which is close to London City Airport.
The fire comes a day after the UK recorded its hottest ever temperatures and firefighters faced their busiest day since the Second World War.
The London Fire Brigade said the brigade’s 999 control officers have taken more than 105 calls about the blaze.
One resident forced to flee the flats said they feared the blaze could be on the scale of the Grenfell disaster.
“The first thing we all thought was Grenfell Tower, we thought we were there, you know?” the woman, who declined to leave her name, said.
As she headed away from her home, she added: “I’m trying to find out where I’m going to go tonight with my family.”
Another resident of the flats said he was forced to alert his neighbours because the fire that started on the 17th floor did not trigger an alarm.
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Mohammad Rajib Hossain, 29, said he only heard about the fire because one of his friends in a different building rang him to say she could see flames.
He continued: “I had to knock on the door, you know, they’re my neighbours, I had to knock the door and say there’s a fire.
“They didn’t know, some of them were sleeping. No one knows, there’s no fire alarm or nothing.
“There is no fire alarm in the whole building, it is inside the flat but not outside.”