Fire crews have battled hundreds of wildfires across the country as temperatures topping 40C transformed fields and grassland into a “tinderbox”.

One regional service said the number of open fires could have tripled this week, while the Met Office warned that most of England was at exceptional risk.

Some were forced to evacuate their homes, and the National Trust admitted it was on “tenterhooks” over the possibility of a large-scale blaze.

On Monday, Essex County Fire and Rescue Service received three times its average number of calls, which a spokesperson said was driven by wildfire reports.

In neighbouring Hertfordshire, drone footage showed the smouldering remains of a 200-acre field after crews worked into the early hours to extinguish the blaze.

One blaze, at Lickey Hills Country Park near Birmingham, spread to around 50,000 square metres and forced 15 people to flee their homes.

Its cause has not been formally established, but a post on the park’s Facebook page claimed it was sparked by a disposable barbecue.

West Midlands Fire Service received 717 incident calls on Monday – an increase of 280 in the space of a week.

Smoke drifted over the M25 as almost 200 firefighters and 30 fire engines tried to extinguish a corn field blaze in Upminster, East London.

Hereford & Worcester crews had 54 fires based around fields, undergrowth and woodland, which it said was a “much higher number” than normal.

Nottinghamshire Fire and Rescue Service has seen a “significant increase” in grass fires over the last month, with numbers rising even further this week.

Charles Spencer, who lives on the Althorp Estate in Northamptonshire, tweeted a picture of one of his fields on fire.

According to the Met Office, most of England is at “exceptional” risk of wildfires, aside from the South West and patches of the North.

“I haven’t even seen the exceptional category being used in the times I’ve looked at the Wildfire Index before,” Kathryn Brown, The Wildlife Trust’s director for climate action, told PA news agency.

“We need to be much more aware, and the public needs to be much more aware, of the wildfire risk.”

Although Tuesday was Britain’s hottest day on record, it is rare for wildfires to start without some sort of human intervention.

The National Trust has appealed to people to avoid lighting barbecues or campfires, and not to leave glass bottles on the ground.

“We’re on tenterhooks that someone is careless with a cigarette butt or thoughtless by lighting barbecues, and us having to deal with a wildfire,” the Trust’s conservation head Ben McCarthy said.

Speaking in Parliament on Monday, Colne Valley MP Jason McCartney warned that the moors in his Pennines constituency were “an absolute tinderbox… at the moment”.