The hottest day of the year so far has been declared with a temperature of 33.3C recorded in Suffolk.
It comes amid we weeks of scorching weather that were due to continue this week - leading the Met Office to issue a heatwave warning.
People should stay out of the sun, meteorologists said. Temperatures are expected to exceed 30C all week.
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Temperatures are expected to be well in excess of 30C this week, with forecasters near-certain that the mercury will top a stifling 34.5C in the east or south-east of England at some point.
People should stay out of the sun if possible, the Met Office has said.
Berries are ripening much earlier than usual due to the searing heat, conservationists have said.
The first ripe sloe was reported to the Woodland Trust's crowdsourcing tool - the Nature Calendar - on 12 July, two months ahead of the baseline average date of 19 September.
The dry weather could spell danger for fruit crops in hedgerows and woods, and for the animals that feed on them, the trust warned.
And leaves on trees may begin to change colour early as they try to preserve water, and they may also be more susceptible to threats such as tree pests and disease
The Met Office said several days of hot weather could "go bang" with the arrival of thunderstorms at the end of the week.
Not everyone will be forced to lie immobile in the shade next to a fan while drinking iced water this week, the Met Office has suggested.
Monday is the hottest day of the year so far, the Met Office has just announced.
A temperature of 33.3C was recorded at Santon Downham in Suffolk.
Even those readers with a sunny disposition might quail at news that the Japanese are also in the midst of a heatwave - and have notched a record-high temperature of 41.1C.
The reading was made north-west of Tokyo in the city of Kumagaya.
New deaths are being reported nearly daily.
Temperatures in Japan just hit a historic new record of 41.1CTemperatures in Japan have reached 41.1C, a new record for the country, amid a deadly heatwave that has claimed dozens of lives. The temperature was recorded in the city of Kumagaya, a city northwest of Tokyo, beating a previous high of 41C in the western prefecture of Kochi in August 2013, according to the Japan Meteorological Agency. Records go back to varying dates for different cities, with data for Kumagaya starting in 1896.
The NHS has issued guidance for staying safe during the hot weather.
High on the list is taking extra care to look after elderly and very young people.
It's not only humans who need to take care during the heatwave - pets can also fall ill as the mercury rises.
Animal charity Cats Protection has offered some advice on how to keep your four-legged friends healthy in the warmer weather.
The weather has even become unbearable for some in sunnier climes than Britain this afternoon - officials in Athens have closed the ancient Acropolis due to heatwave conditions.
Workers shut the site to tourists at around 2pm local time due to Greek laws permitting public site to close if temperatures reach 36C.
The Acropolis is Greece’s most popular ancient attraction, with more than 2.7 million visitors each year.
The Environment Agency has warned this summer’s extended heatwave has taken its toll on the UK’s nature.
Officials have responded to 44 “significant” environmental incidents since the end of June, including moorland fires, algal blooms, dry boreholes, low river flows and fish rescues as the hot, dry weather continues to grip.
The EA said it had stepped up its incident response to tackle the impacts of the ongoing dry weather, following the driest June since 1925, which is putting pressure on wildlife as rivers dry up.
In the North West of England, the lack of rain has led to a rapid decline in reservoir levels, while water company United Utilities has announced a hosepipe ban for 8 million customers from early August.
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