A four-storey block of flats in a Lincolnshire town was evacuated after a blaze engulfed the top floor.
At one point, firefighters had to leave the inside of the building in Boston after it began to collapse.
The blaze, on Charleston Terrace, was believed to have started on Tuesday afternoon in a cavity and spread to the roof, the fire service said.
There were no reports of any injuries and residents who fled their homes were given water by staff at a nearby store.
A total of eight fire crews, including some using aerial platforms, attended the blaze from across the county after being called out at about 13:55 BST.
Incident commander Lee Marsh, from Lincolnshire Fire and Rescue Service, said firefighters would remain at the scene to ensure the building was safe.
“There was a fire in a flat, but unfortunately it escalated and it has affected the entire block,” he said.
“We are just waiting for building control to come back and help us and give us some advice on how stable that building is.”
The road and a nearby rail line had been closed due to the “significant amount of damage” to the building, he added.
“Our concern is that any further debris falls into the road or falls along the train line that services the port, so we asked for that to be stopped for the time being,” he said.
One eyewitness told the BBC he had never seen anything like it in Boston before.
“I just saw a fire on the roof. It was still going after five hours – just lots of fire and smoke,” he said.
The fire was one of many across Lincolnshire as the county saw record temperatures on Tuesday.
The Met Office said Coningsby in Lincolnshire reached 40.3C (104F) – the highest UK temperature ever recorded.
Emergency services in Lincolnshire declared a major incident as crews battled a number of wildfires and firefighters from neighbouring counties were called in to help.
A number of fire crews tackled a large fire in a fields at Gayton le Marsh, near Alford.
Lincolnshire Fire and Rescue Service said it had now stood down the major incident, but crews remained at a the scene in a number of locations across the county.
Source: BBC