Britain was sweltering Thursday in its first prolonged heatwave in seven years as the deputy prime minister warned the country was simply "not ready for this".

Britain experienced its hottest day of the year so far on Wednesday, with thermometers hitting 32.2 degrees Celsius (90 degrees Fahrenheit) in southwest London.

The Met Office national weather service put the southwest of England and the west Midlands on the level 3 -- one grade below the "national emergency" top level -- joining London and the wider southeast on heatwave alert.

"This stage requires social and healthcare services to target specific actions at high-risk groups," it said.

Forecasters have said Britain is in the midst of its first prolonged heatwave since 2006, with six consecutive days of temperatures above 30C (86F).

Amid stifling temperatures on public transport, Deputy Prime Minister Nick Clegg told London radio station LBC that Britain was simply "not ready for this".

"The problem we face as a country is these things are unpredictable," he said on the variable British weather.

"The whole way we work -- our offices, buses, everything -- isn't actually designed for these sudden spikes in temperature.

"In countries that are used to hot temperatures they organise themselves differently, or indeed very cold winters."

Responding to reports that temperatures on London's Underground trains had topped 35C (95F), Clegg said that was "pretty brutal" and "unbelievably high".

Research conducted for The Times newspaper by the London School of Hygiene and Tropical Medicine estimated the death toll for the first nine days of the current hot spell in England as between 540 and 760 people.

"Almost everywhere in the world when it gets too hot for that place, deaths will go up in the few days after," the university's Professor Ben Armstrong said.

The hottest temperature ever recorded in Britain was the 38.5C (101.3F) reached in Faversham, southeast of London, in 2003.

As the nation waits for Prince William's wife Catherine to give birth any day now, Public Health England (PHE) warned that pregnant women may experience discomfort in the heatwave.

"High temperatures can be dangerous, especially for people who may be vulnerable," warned Doctor Angie Bone, heatwave plan leader for the PHE, a branch of the Department of Health.

"For those working or exercising outdoors, strenuous physical exertion during the hottest part of the day should be kept to a minimum," she added.

This month could be one of the driest Julys in nearly 300 years.

At Reading Crown Court, west of London, where the metal roof contributed to temperatures of 30C (86F), judge Peter Ross ditched his robes and wig and sat in judgement in a linen jacket instead, The Times said.