A punishing heatwave has broken a string of records in France and Britain, as Spain battled to contain forest fires that forced hundreds from their homes.

The hot weather is in line with warnings from scientists that heatwaves will be more intense and hit earlier than usual due to climate change.

In Spain, forest fires burned up to 7,000 hectares of land in the northwest Sierra de la Culebra region, forcing some 200 people from their homes.

Firefighters were battling fires in several other regions, including woodlands in Catalonia, where weather conditions complicated the fight.

Prime Minister Pedro Sanchez praised firefighters "who risk their lives on the frontline of fires".

Temperatures were above 35C in most parts of the country and will top 40C in some areas.

In France, temperatures also caused around one third of French departments to activate the highest or second-highest heat alert level.

French Red Cross distribute bottled water to the homeless in Toulouse

Much of the country's southwest will see thermometers top 40C, prompting warnings for the vulnerable.

"Hospitals are at capacity, but are keeping up with demand," Health Minister Brigitte Bourguignon told reporters in Vienne, near Lyon in the southeast.

Schoolchildren have been told to stay at home in the 12 departments at alert level red and the health ministry activated a special heatwave hotline.

Special measures have been taken in care homes for elderly people, still marked by the memory of a deadly 2003 heatwave.

Buildings are being sprayed down with water to cool them and residents are being rotated through air-conditioned rooms.

"We're taking even more care than usual with the old folks. It's tough for them. They're often alone, worn down physically and unable to act independently," said Sarah Jalabert, a nurse making home visits in the Tarn department.

In the Gironde department, which includes Bordeaux, authorities said all public events outdoors or in non-air-conditioned venues are banned from 2pm, a measure set to be broadened across the region.

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Speed limits in several regions, including around Paris, have been reduced to limit the concentration of harmful smog or ozone in the heat.

"This is the earliest heatwave ever recorded in France" since 1947, said Matthieu Sorel, a climatologist at weather authority Meteo France.

With "many monthly or even all-time temperature records likely to be beaten in several regions," he called the unseasonable weather a "marker of climate change".

The heatwave has spread up from north Africa through Spain, also affecting Italy and the UK.

Several towns in northern Italy have announced water rationing and the Lombardy region may declare a state of emergency, as a record drought threatens harvests.

A sign in a London Underground station today

The UK has recorded its hottest day of the year, with temperatures reaching over 30C in the early afternoon, meteorologists said.

It was the third day in a row that temperature records had been broken in the UK, where it was over 28C on Wednesday and 29.5C yesterday.

Read more: Europe's record hot summers in past two decades

Experts warned that the high temperatures were caused by worrying climate change trends.

"As a result of climate change, heatwaves are starting earlier," said Clare Nullis, a spokeswoman for the World Meteorological Organisation in Geneva.

"What we're witnessing today is unfortunately a foretaste of the future" if concentrations of greenhouse gases in the atmosphere continue to rise and push global warming towards 2C from pre-industrial levels, she added.