uk heatwave
People sit by a pond at Marble Arch, in central London.Reuters

Britain basks in sweltering heat and humidity, with health warnings issued to vulnerable groups.

The elderly, the young and the sick are advised to keep cool and stay out of the sun.

Dr Paul Cosford, director for health protection at Public Health England (PHE), told the Met Office: "Older people and those with long-term illnesses are particularly vulnerable to the effects of very hot weather, so it's important to look out for them, help them stay hydrated with plenty of cool drinks, and keep indoor areas as cool as possible.

"Try to keep bedroom and living spaces cool, by closing the curtains on windows that receive the sun and open your windows at cooler times of the day and overnight. A trip to a park where there's lots of shade or a public building that is air conditioned, may help offer some relief from the heat for vulnerable people."

The blistering temperatures are due to tropical continental air from Iberia and Spain, weather forecaster Marco Petagna told the Daily Express.

He also warned: "There is also the potential for pollution levels to start increasing and we are expecting very high pollen levels with grass and week pollen levels peaking at the moment."

Leon Brown, forecaster for The Weather Channel UK, said tropical conditions will trigger violent thunderstorms later in the week.

He said: "The west is a little cooler on Thursday with a line of thunderstorms likely to run northwards over central Britain while the east of England remains hot and sunny.

"There is the potential for some local very heavy downpours over central and western England and flash flooding.

"Friday looks like another very hot, humid and mostly dry day over central and eastern Britain with temperatures again breaking 30C widely and could top 33C.

"There is a change from the west next weekend, but the heat may hang on across the east on Saturday."

Temperatures at Wimbledon could also reach an all-time high this week, possibly exceeding the record 34.6C set in 1976.

The All England Club's Heat Rule, which allows women players to take breaks, comes into effect when the heat stress index - a measure that factors in air temperature, surface temperature and humidity - exceeds 30.1C.