Climate demonstrators on Wednesday triggered a lengthy tailback on Britain's busiest motorway, warning that a record-breaking heatwave was a dire reminder for urgent action.

Members of the group Just Stop Oil climbed gantries over the M25 encircling London, causing police to intervene and vehicles to back up for several miles (kilometres) in one direction.

Surrey Police later said a 22-year-old woman who had climbed a gantry was arrested on suspicion of causing a danger to road users, causing a public nuisance and for being a pedestrian on the motorway.

Three lanes that were shut as she was brought down were later reopened, the force added.

Temperatures topped 40 degrees Celsius (104 degrees Fahrenheit) in southern and eastern England for the first time on Tuesday, with a new record set at 40.3C.

The extreme heat caused huge transport disruption and sparked the same kind of wildfires seen in Europe in recent years.

Government minister Kit Malthouse told parliament that 34 places saw temperatures in excess of the previous record of 38.7C in 2019.

Firefighters saw "their busiest day since World War II", dealing with dozens of wildfires as 15 fire and rescue services declared major incidents, he said.

Dozens of wildfires broke out across England on Tuesday due to a fierce heatwave that pushed temperatures to record levels
Dozens of wildfires broke out across England on Tuesday due to a fierce heatwave that pushed temperatures to record levels AFP / William EDWARDS

Malthouse revealed at least 41 properties were destroyed in London, 14 in Norfolk, eastern England, five in Lincolnshire, and smaller numbers elsewhere.

The minister, who chaired recent meetings of the government's emergency contingencies committee, urged the public to heed safety advice, as the risk of more wildfires was still high despite temperatures easing.

"Tragically... 13 people are believed to have lost their lives after getting into difficulty in rivers, reservoirs and lakes while swimming in recent days -- seven of them teenage boys," he added.

London Fire Brigade said 16 firefighters were injured around the capital with two taken to hospital.

Assistant Commissioner Jonathan Smith said the service was currently updating risk management plans for the capital, with climate change "front and centre" in the process.

"We're dealing with extremes at both ends, whether it's extreme heat or whether it's extremes as far as floodwater is concerned," he said during a visit to Wennington, one of the London areas devastated by fires.

The city's mayor Sadiq Khan said the service received more than 2,600 calls on Tuesday -- up from a normal day of about 350.

Khan also accused Conservative leadership candidates vying to succeed Prime Minister Boris Johnson of ignoring "the elephant in the room" of climate change.

Just Stop Oil said it regretted disruption to the public from its latest action on the M25, after activists had previously staged sit-in protests on that road and others.

But declaring the M25 "a site of civil resistance", it warned of further protests to come this week.

"This is the moment when climate inaction is truly revealed in all its murderous glory for everyone to see: as an elite-driven death project that will extinguish all life if we let it," the activist group said.