Smashing previous records, the United Kingdom logged its highest temperature for June, reaching 36.1C in southern England as a heat dome hovered over much of western Europe.
Temperatures in Paris hit a June record of 40.9C, a day after France recorded its hottest day since records began nearly 80 years ago when temperatures peaked at 44.3 C in the southwestern town of Pissos.
Italy's health ministry placed 16 cities - including Florence, Milan, Rome, Turin and Verona - on its highest heat alert, and warned the heatwave could intensify further, peaking between Sunday and Monday.
At least 48 people have died in France from drowning since the onset of the heatwave while trying to cool off, authorities said, and two young children were killed by heat in a car.
Spain reported two elderly people had died of heat stroke after days of temperatures exceeding 40C although conditions there began to ease on Wednesday following the hottest late-June days on record, according to weather agency AEMET.
Scorching temperatures killed hundreds of thousands of birds at poultry farms in Brittany and the Pays de la Loire, agricultural groups said.
France's nuclear power plants, which supply most of the country's electricity, cut output by about seven per cent of total demand as high temperatures limited access to cooling water.
The heatwave is being driven by a weather pattern known as an Omega block, pushing temperatures as much as 18C above normal, according to the Reuters Climate Monitor.
The phenomenon resembles the shape of the Greek letter Omega, with a bulbous middle trapping in heat over regions for extended periods, with cooler weather on its fringes.
In the UK, which has a reputation for being grey and drizzly even in summer, the heat was particularly uncomfortable, not least because so much of the country's infrastructure - such as buildings and transportation systems - was built for cooler weather.
Hundreds of schools closed or shortened their day as officials warned that high temperatures could endanger even healthy people.
The 36.1C reported by the Met Office in Gosport in Hampshire edged above the previous June record of 35.6C set in 1957 and matched in 1976.
Even London Climate Action Week was disrupted, with organisers cancelling an event on extreme heat because of the heat itself.
The city's rail networks were affected by the heat, including delays and speed restrictions on major London Underground lines and cancellations of some commuter services.
Some trains in Wales were also cancelled due to extreme heat.
Weather agency Meteo-France has said the conditions are comparable to a heatwave in August 2003 that lasted 16 days and caused an estimated 80,000 excess deaths across Europe.
Conditions in France were expected to remain stifling on Thursday, the forecaster said, extending a red alert weather warning to 72 districts across the country.
Europe is warming at more than twice the global average, the World Meteorological Organisation has said, making prolonged heat episodes increasingly likely.
The Eiffel Tower and the Louvre announced early closing times and the Changing of the Guard outside Buckingham Palace was scaled back, without the usual ceremony of soldiers in scarlet tunics and heavy bearskin hats.
The Uffizi Galleries in Florence, one of Italy's top tourist sites, halted ticket sales on Wednesday to fix an air conditioning malfunction.
In Switzerland, local authorities opened air-conditioned theatres for free daytime cinema screenings.
A French agricultural co-operative said farmers had moved to night shifts to protect workers and reduce fire risk.
with AP
