• Met Office triggers measures to help vulnerable and elderly
• Tomorrow could be hottest before weather cools at weekend

A heatwave has been declared in Britain, with temperatures in southern England expected to have hit a sweltering 32C (89.6F) – even hotter than yesterday.

The Met Office raised its heatwave plan to level three, or amber alert, in London and south-east England, triggering measures designed to help safeguard the welfare of thousands of elderly and other vulnerable people at risk from heat-related conditions.

Yesterday was the hottest day of the year to date, with temperatures reaching 31.8C (89.24F) in Wisley, Surrey, as June became the hottest month in three years.

The chief forecaster at the Met Office, Eddy Carroll, said: “Temperatures are likely to peak today and tomorrow, reaching 32C before becoming less hot by the weekend as noticeably fresher conditions with the risk of showers push east across the country.”

The Department of Health warned that temperatures could be higher tomorrow, touching 33C (91.4F), and advised people with respiratory problems to stay inside during the hottest parts of the day.

A heatwave is officially declared when the Met Office confirms that the threshold of an average temperature of 30C by day and 15C overnight for one or more regions has been met and there is a 90% risk that the daytime threshold temperature will be met the following day.

Parts of England have seen temperatures climb to highs of around 31C by day and in some areas they have not fallen below 18C at night.

The head of health forecasting at the Met Office, Wayne Elliott, said: “There are four key things to try to 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 and babies and young children.”

Yvonne Doyle, the director of public health in south-east England, 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.”

Temperatures in the capital have been higher than in Bangkok this week, and Trafalgar Square’s fountains turned green as algae spread.

There is an 80% risk of heatwave conditions in the East and West Midlands and a 70% chance in eastern England. Temperatures are cooler in the north of England, where the chance of heatwave conditions is just 40%. The areas least at risk are Wales, where there is a 20% risk, and south-west England, where there is only a 10% chance of heatwave conditions.

Only extreme coastal areas, Northern Ireland and the west coast of Scotland, where temperatures were hitting a more moderate 20C (68F), escaped today’s heat, said a Met Office spokeswoman. Average temperatures for the time of year, which normally hit about 20C, were being surpassed, with most places recording more than 25C (77F), she said.

The weather has prompted the government to set up a heatwave advice page on its Directgov website, while the NHS Direct helpline has received hundreds of calls from patients suffering symptoms related to the heat.

The London ambulance service said it 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 – and urged people to call only in a genuine emergency.

Staff should be encouraged to wear shorts during the sweltering heat to make work more bearable and prevent them “collapsing” at their desks, the TUC urged. The weather has sent sales of electrical fans and ice cream soaring, according to Tesco and Sainsbury’s.

Keepers at London zoo have been feeding gorillas ice blocks, while tapirs at Port Lympne wild animal park near Ashford, Kent, were daubed with suncream by staff yesterday.

A London zoo spokeswoman said: “In this really hot weather we give the gorillas ice lollies which we make in buckets. We put fruit inside the ice lollies which they try to get out by standing on the blocks. Our gorilla group also have air conditioning inside their enclosure so they can go inside and outside into the heat as they please.”

The highest level of Britain’s heatwave plan is level 4, the red or emergency alert, which is declared when a heatwave is so severe or prolonged that the general population, and not just vulnerable groups, is at risk from heat-related conditions.

Social services and hospitals on alert for hot weather casualties, with no relief from heat expected until weekend

An official heatwave health warning will be issued tomorrow morning, meaning that schools, hospitals, local authorities and social services must take action to avoid casualties, unless temperatures plummet tonight.

London and much of the south-east has already sweltered through two days where temperatures topped 30C, and tonight temperatures in the low 20s are expected in many areas. The lowest temperature anywhere in the Greater London area on Monday night was 18C.

The temperature hit 31C today at Heathrow. The highest temperatures recorded so far this week were 31.4C at Wisley in Surrey, and 30.4C in St James’s Park, London, both on Monday.

The scorching end to the month made June the hottest, driest and sunniest month since July 2006. Ladbrokes has cut the odds on a record-breaking UK summer from 9-2 to 4-1.

According to climatologist Philip Eden, England and Wales had on average 222 hours of sunshine in June, 117% of the figure for 1971-2000, while rainfall was only 80% of the same period.

Already this week Tesco reports that sales of electrical fans are 20 times that of last week, and they expect to sell 100,000 before any relief from the heat is forecast at the weekend.

Tomorrow could be the hottest day of the year, with temperatures predicted to reach 33C.

The Met Office has issued a warning of heavy rain across most of Wales this evening, and advance warnings of thundery downpours in the west tomorrow which could cause localised flooding.

The hottest spell in three years is already leaving the elderly, very young, frail and those with breathing problems particularly vulnerable. Advice on health, travel, and how to manage the heat in homes, schools and workplaces is available on the government’s Directgov website, drawing together information from the Met Office, NHS, and the Highways Agency.

During record temperatures in 2003, an estimated 2,000 people died from heat-related conditions.

NHS Direct and the London ambulance service, which will bear the brunt of pressure in the capital, are gearing up for a major demand on their services.

The UK’s heatwave plan remains at level two, but the Met Office believes it will be raised to level three.

Official advice says anyone who can should stay indoors from noon to 3pm, draw curtains to shade rooms, drink plenty of water and wear sunscreen while outdoors.

Many schools are already keeping children out of playgrounds during breaktimes.

Care homes have been advised to monitor indoor temperatures four times a day, identify high-risk residents and prepare a cool room.

Officials are urging people to identify the coolest room in their home, stock up on essential supplies to cut down on shopping trips and check on neighbours, relatives and friends, especially those with mobility problems.

The Department of Health and the lifeboat service have warned people to be careful if going swimming.

A teenage boy, who has not yet been formally identified, drowned while swimming with three friends in the river Severn yesterday.

Search teams using a helicopter and boat recovered the boy’s body. He was confirmed dead on arrival at Worcestershire Royal hospital.