Temperatures had already reached 30C (86F) at Heathrow Airport in London by mid afternoon, with today set to be the hottest of the year so far.

The Met Office said the hot spell, which has now extended over nearly a week, was being experienced across the country.

Only extreme coastal areas, Northern Ireland and the west coast of Scotland, where temperatures were hitting a more moderate 20C (68F), have escaped today's burning heat, said a Met Office spokeswoman.

She said average temperatures for the time of year, which normally hit about 20C, were being surpassed, with most places recording more than 25C (77F).

The Met Office said the warning level was raised to three, one below the highest level, because high temperatures had been reached and sustained in one or more regions, with two consecutive very warm nights and a hot intervening day.

Those English regions affected by the heatwave and at level three include London and the south east.

The spokeswoman said the current weather spell was the hottest since July 2006, with yesterday the hottest of the year so far at 31.8C (89.2F).

The heatwave plan alert is in four stages, with green level one signalling "summer preparedness and long-term planning".

Level two is amber and signals "alert and readiness", while three is red for "heatwave action".

Level four is classed as "red emergency".

Wayne Elliott, head of health forecasting at the Met Office, said: "There are four key things to try and remember during a heatwave.

"If possible stay out of the heat during the middle part of the day, cool yourself down, keep your environment cool and look out for others, especially older people, those living alone, babies and young children."

Yvonne Doyle, the Government's regional director of public health in the South East, said: "Everyone's health can be affected by the heat, with greater risks of dehydration, sunburn, heat exhaustion and heatstroke.

"Those most at risk are the very young and the very old, and people who already have health conditions, especially heart and respiratory problems - the heat can make their symptoms worse."

The London Ambulance Service said it had seen a busy few days and urged people to only call for an ambulance in a genuine emergency.

It has been treating large numbers of patients for breathing problems, chest pains, loss of consciousness and fainting.

The service received 4,765 calls yesterday - an increase of 21% on the previous Tuesday.

A total of 10,605 calls were taken on Saturday and Sunday - an increase of nearly a fifth on the previous weekend.

On Monday, 5,087 calls were received. Of these, 226 were for breathing problems, up from 160 on the same day in the previous week.

The number of calls regarding fainting rose from 191 from 158.

Deputy director of operations Jason Killens said: "Anyone with minor conditions should consider other healthcare options, such as calling NHS Direct on 0845 46 47 or visiting a minor injuries unit, NHS walk-in centre or local pharmacist.

"If they need to go to hospital for non-emergency treatment, they should use a taxi or get a lift from a friend or relative."

The latest figures from NHS Direct showed there were 318 calls on Tuesday from people with heat-related symptoms - down on the number this time last year.

The company, FirstCare, which works with 17 NHS trusts, said absence rates for NHS staff rose by around a quarter on Monday compared with the previous week.

Many calls were received from people after they had started work, suggesting the heat had an effect on their health, it said.

FirstCare monitors absence rates for around 27,000 employees.

The number of NHS employees absent on Monday was 3.82 in every 1,000, compared with 2.96 the week before.

Aaron Ross, chairman of FirstCare, said: "The weather always has an effect on absenteeism.

"What we saw was quite unusual - there was a significant increase in the number of NHS workers calling in sick, but many of them were calling us after they had gone to work.

"This indicates that the working conditions in the extreme heat had a negative effect on their overall health, causing them to go home.

"Whilst we also saw an increase in the number of office workers it was much less significant at only 16% higher than normal.

"We believe this is largely due to the fact that they are generally more comfortable at work due to large open spaces and air conditioning."

The Dogs Trust urged people not to leave their pets in a car in warm weather.

"Just 20 minutes waiting in a hot car can prove fatal," a spokesman said.

The charity also suggested walking dogs in the morning and evening, when the weather is cooler, and making sure they have drinking water.

Older and overweight dogs are also more prone to overheating, it added.