Temperatures in Britain are set to reach their highest-ever levels on Thursday as northern Europe baked in a heatwave for the second time in a month.

The Met Office, Britain's national weather forecaster, said there was a 40% chance that the record of 38.5 degrees Celsius (101.3°F), set in Kent in 2003, will be broken.

A spectator uses a fan to keep cool at Lord's cricket ground in London on Thursday.

Britons were facing travel disruption, with trains being forced to slow down to prevent tracks buckling in the heat, and health authorities have issued warnings to the vulnerable.

High pressure drawing scorching air from the Sahara desert has already broken temperature records for Belgium and the Netherlands and is expected to persist until Friday.

Climate specialists warned that such heatwaves were becoming more frequent as a result of global warming from greenhouse gas emissions.

A Met Office study found that last year’s heatwave was 30 times more likely to occur than in 1750 because of the high amount of carbon dioxide in the atmosphere. Since the pre-industrial period (1850-1900) the Earth’s surface temperature has risen by 1 degree Celsius.

"There is a 40-50% chance that this will be the warmest July on record. This heatwave is exactly in line with climate change predictions," said Dr Karsten Haustein at the Environmental Change Institute at the University of Oxford.

A mother and child cool off by walking through a park fountain in Antwerp, Belgium, on Thursday.

Peter Inness, Senior Research Fellow at the University of Reading, said: "The fact that so many recent years have had very high summer temperatures both globally and across Europe is very much in line with what we expect from man-made global warming."

The heatwave in Britain is expected to come to an abrupt end on Friday with thunderstorms forecast for several parts of the country, the Met Office said.

Belgium hit record high of 40.2c

Official meteorologists registered a record high temperature for Belgium on Wednesday as western Europe sweltered under a heatwave that has disrupted transport and triggered safety warnings.

A worker stops to drink from a bottle of water in Madrid, Spain, on Thursday.

David Dehenauw, chief forecaster at the Royal Meteorological Institute, said Thursday that a high of 40.2 degrees Celsius (104 Fahrenheit) had been recorded the day before in the eastern city of Liege — the highest ever since records began in 1833.

Due to the high temperatures, the institute issued a "red alert" for almost all of Belgium -- except the North Sea coast — through Friday.

In Brussels on Thursday the Royal Meteorological Institute measured 34.2 degrees at noon (10:00 GMT), where Wednesday's 35.7 degrees looked set to be exceeded in the afternoon.

Agencies