The weather may have made Britain feel like a tropical jungle for much of this week, but there could be even steamier heatwaves to come.

As most of the country was sweltering yesterday, with the temperature in London higher than in Bangkok, NHS Direct received 378 calls from people suffering symptoms related to the heat. More than 100 related to insect bites, 69 were about breathing difficulties and 21 were about fainting. The weather even prompted the Government to set up a heatwave advice page on its Directgov website.

Records show that the past month has been the hottest, driest and sunniest since July 2006. David Whiffen, the ice cream buyer for Sainsbury’s, said: “We’re expecting the best week for ice cream since the heat wave of 2006.”

He added that ice cream sales in the company’s outlets had doubled compared with the same week last year and had increased by 75 per cent from last week.

Britain’s highest temperatures usually come in late July or early August, when the seas have reached their warmest. If the ground also becomes dry, it behaves like a furnace and sends temperatures even higher. If hot air drifts across from Europe, serious heat can build up. This happened in the heatwave of August 2003 when the highest temperature in Britain was recorded — 38.5C (101.3F) at Brogdale, Kent.

Yesterday the Met Office reiterated its long-range forecast for a “barbecue summer” of above-average temperatures. Provisional figures up to June 28 show that temperatures across Britain were 0.7C above normal.

“We were expecting spells of weather hitting 30C, and that’s come true,” said Helen Chivers, from the Met Office.

It has also been a dry month, with 59.5mm of rain, 82 per cent of normal, and 198 hours of sunshine, or 17 per cent above average.

However, the temperature and sunshine figures are expected to rise higher once the last two days of June have been included. Even with some fresher weather on the way in the next day or two, temperatures are still expected to be near average for the time of year.

The key date to look out for now is St Swithin’s Day, July 15. In folklore, if it rains on this saint’s day the weather will be wet for the next 40 days, but, if it is dry, then 40 days of sunny weather should follow.

Although the prediction never strictly comes true, this ninth-century bishop and his long-range forecast has some sound science behind it.

Around the middle of July the weather in Britain tends to settle into a pattern that can last into August, which is true for about seven out of ten summers. This persistent weather is caused by the jet stream, a ribbon of wind that blows high around the globe, formed at the battlefront between Arctic and tropical air.

In high summer, the position of the jet stream tends to become fixed. If it takes a southerly track then Arctic air can sweep down and make a miserable summer, and when the jet stream is close to Britain it drags wet depressions off the Atlantic — which is what made the summers of 2007 and last year so bad. When the jet stream flows north, Britain basks in fine, warm weather. This lends some credibility to the St Swithin proposition.

Economists say that productivity could drop by up to a third as staff take unauthorised days off to enjoy the sun. Hundreds of thousands of workers are expected to have taken at least one “sick day” by the end of the week.

The Centre for Economics and Business Research said that a repeat of the sweltering temperatures over the next few days would result in lost productivity worth up to £162 million every day.