Devastating wildfires have ripped through parts of the country as the longest heatwave for seven years spread across Britain and forecasters warned temperatures could climb as high as 35C.

Mountain blazes tore across the south Wales valleys while flames devastated swathes of Tentsmuir Forest in north-east Fife, Scotland, on Thursday night, and London experienced its worst grass fires since 2006.

The spate of hot weather is believed to have caused up to 760 premature deaths already and weathermen warned that the hottest day of the year is yet to come.

The mercury - which reached reached 32.2C (90F) on Wednesday - is expected to rise to around 33C next week. Weathermen said there was a "slim" chance it could even hit 35C in the South on Tuesday or Wednesday.

Paul Mott, a forecaster with MeteoGroup, the weather division of the Press Association, said temperatures would cool slightly on Saturday in many areas but could reach 28C in the Highlands. "It looks likely that the heatwave will return in the early part of next week when temperatures will be back in the low 30s," he said. The weather is then expected to become more humid, bringing showers and thunderstorms in the West, between Monday and Wednesday, with the possibility that the mercury could rise to 35C. "There is a slim chance that we could see temperatures much higher than we have done," Mr Mott said. But they are unlikely to top the high of 36.5C recorded in Surrey in July 2006.

The Met Office has warned of an "elevated risk" of fires in the countryside following six consecutive days of plus-30C temperatures and a dramatic reduction in the average monthly rainfall. Crops due for imminent harvest are said to be particularly vulnerable to blazes.

Meanwhile police in Lincolnshire reported another death on Friday afternoon, after a woman was pulled from the sea in Skegness. Emergency services were called to the beach area behind the Beachcomber on Roman Bank, Ingoldmells, shortly after midday. Officers said the 69-year-old woman's death was not being treated as suspicious. The coroner has been informed, they said. No further details about the woman have been released.

The British Red Cross has launched two call centres in Norwich and Ipswich to check on the welfare of hundreds of elderly and vulnerable people in the region. Teams of volunteers are working with hospitals across the area to ensure that those who have recently been discharged and have been identified as needing extra care are coping in the heatwave.

A spokesman for the RAC said staff have been called out to 13,000 breakdowns because of the heatwave - an average of 7,163 each day. Simon Williams, of the RAC, said: "The heat is causing massive issues for drivers. We have been 50% busier in the evenings as people make the most of the weather for as long as they can. Demand has also switched from towns and cities to coastal routes and motorways. We're dealing with more mechanical breakdowns where cars conk out on the move. There's also been a spike in the number of people running out of fuel. Some of these breakdowns are avoidable through maintenance and preparation, but even great servicing can't stop your head gasket blowing."

Insurers have warned wildfires could put lives at risk and cost millions of pounds in damaged crops and machinery. Tim Price, of NFU Mutual, said: "The tinder-dry conditions and continuing heatwave pose a major fire risk to the countryside, threatening crops, equipment and even personal safety. Every precaution needs to be taken by both farmers and visitors alike." Farmers have been urged to ensure that their fire extinguishers are well maintained and to reduce the risk of combine harvesters catching fire by regularly cleaning the machinery to remove chaff and dust.