British authorities issued a heatwave alert on Wednesday after the first prolonged period of high temperatures since 2006, following the coldest spring for more than 50 years.

The country has been basking in sunshine for several weeks and temperatures have reached more than 30 degrees Celsius (86 degrees Fahrenheit) for five days in a row for the first time in seven years.

In a nation famed for its rainy climate and grey skies, temperatures were set to hit 32 degrees Celsius (90 degrees Fahrenheit) in parts of England on Wednesday, with the sunny weather forecast to continue.

Most Britons have been enjoying the heat after a washout spring, packing beaches and parks, and retailers have reported strong sales of barbecues, paddling pools and garden furtniture.

But health authorities have also issued warnings about the dangers posed by the heat to old people, young children and those with serious illnesses.

The Met Office declared a heatwave alert for London and the southeast of England, prompting hospitals, care homes and community health workers to step up checks on vulnerable patients.

On Monday, a woman in her 80s was hospitalised in Cornwall, southwest England, after becoming stuck in her deckchair in the baking sun all day.

Firefighters were called to rescue her from the broken chair after a neighbour raised the alarm.

Police have also urged those looking to cool off to be careful about swimming in open water, after a number of deaths in recent weeks.

The bodies of a 16-year-old boy and a 41-year-old man were found on Tuesday after apparently drowning while swimming in separate lakes in a disused quarry in Norfolk, eastern England.

Two reservist soldiers also died of suspected heatstroke at the weekend during a training session in Wales, when temperatures reached 29.5 Celsius degrees (85.1 degrees Fahrenheit).

It is not just humans who are feeling the heat. Britain's RSPCA animal charity has issued advice on how people can help their pets stay cool as the mercury soars.

"Simple things like moving small animals out of direct sunlight, topping up drinking water and waiting until the cooler part of the day to walk your dog or ride your horse can make a real difference to the welfare of your animals," said RSPCA chief inspector Dermot Murphy.

The heatwave follows a washout British summer in 2012, the coolest since 1998, which raised fears for the London Olympics games.

The record maximum temperature in Britain in July is 36.5 degrees Celsius (97.7 degrees Fahrenheit), set in 2006.