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

Members of the group Just Stop Oil climbed gantries over the M25 arterial motorway, causing police to intervene and vehicles to back up for several kilometres in one direction.

Temperatures topped 40 degrees Celsius in southern England for the first time on Tuesday, as Britain battled huge transport disruption and the same kind of wildfires seen in Europe in recent years.

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

"Yesterday was the busiest day for the fire service in London since the Second World War," the city's mayor Sadiq Khan told Sky News, urging the public to remain vigilant despite temperatures now falling.

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.