Heatwave LATEST: Travel warnings issued as Paris and northern France braced for hottest day ever

As the country gears up to face what is predicted to be a record-breakingly hot day day, French rail operator SNCF has called on passengers only to travel if absolutely necessary.

The company says that SNCF and suburban RER trains across France will still be running, but could be delayed as some stretches of lines will have extra speed restrictions because of the heat.

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Pre-bought tickets can be refunded or exchanged until the end of Thursday. Passengers who do decide to travel are advised to bring bottled water.

Meanwhile Paris commuters on the Paris Est lines are warned that services are very disrupted on Thursday morning because of a fire in an electricity substation.

The blaze, believed to be related to the heatwave, broke out in Noisy le Sec in Seine-Saint-Denis on Wednesday night and brought to a halt all services on the east side of the city – TGV Est, TER, Intercités, and RER lines E and P.

Twenty départements in northern France – including the greater Paris area – are currently on red alert, defined as a temperature that can be harmful to health.

The mercury in Paris was set to reach 42C today which will easily beat its current record temperature of 40.4C set back in 1947.

At 6am Météo France recorded temperatures of 26.5C in Paris and 27.1C in Bordeaux.

The French weather forecaster is predicting that Paris, Lille, Rouen, Orléans, Reims, Dijon, Nancy and Strasbourg will all break their all-time temperature records on Thursday.

Five cities have now introduced restrictions on cars due to high levels of air pollution – Paris, Lille, Lyon, Strasbourg and Annecy.

In Paris only vehicles with CritAir stickers 1 and 2 are allowed, while the others allow stickers 1,2 and 3. In Lille an exception has been made for carpooling vehicles which are allowed into the city regardless of which sticker they have.

France's Health Minister Agnes Buzyn had a stern warning for members of the public who could suffer health problems as a result of the extreme heat: “Everyone is at risk with these kinds of temperatures.”

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She warned that it was ill-advised to take part in any sporting activity over the coming days.

It's only the second time Météo France has issued red level warnings for a heatwave in France. The first time was during June's heatwave when several départements in the south were place on red alert as France recorded its highest ever temperature of 46C.