Half of French drivers have been banned from using their cars in Paris as the city enters its third consecutive day of measures to ease a spike in air pollution.

Public transport has been free since Tuesday in the smog-filled French capital, which is suffering its worst and most prolonged bout of winter pollution in at least 10 years.

Only drivers with even-numbered registration plates are permitted to drive in the Paris region on Thursday, authorities have said, amid fears pollution levels could stay high for days.

Paris make public transport free because of air pollution

This follows similar restrictions yesterday, when only cars with odd-numbered plates were allowed on the roads, and Tuesday, which was even-only – the first time in 20 years the measure has been used three days in a row, according to Le Parisien.

A fog of fine pollution particles descended on Paris over a week ago, triggering a pollution alert when the AirParif agency published readings of more than 80 microgrammes of the pollution particles, known as PM10, per cubic metre of air particles.

The high concentration of the toxic particles in the region is due in part to low winds and high pressure keeping emissions from traffic, power plants and heating – especially log fires – close to the ground.

And air quality monitor AirParif is set to publish a new pollution alert today as levels remain over 80 microgrammes per cubic metre, reported Le Monde.

Paris is not the only French city to be affected by pollution spike – according to the newspaper, Marseille, Avignon, Rouen and Grenoble are also experiencing the low air quality seen in pictures of the Eiffel Tower engulfed in smog.

Traffic on the 'peripherique', the main ring road in Paris, the day before alternate traffic circulation measures were introduced to ease persistent pollution (Getty Images)

Alternate traffic measures will also be implemented from Friday in Lyon in south west France, where a pollution alert is expected to be issued this week.

While Paris has resorted to the policy to ease air pollution levels three times before, in 1997, 2014 and 2015, this is the first time drivers in Lyon will be affected.

However, many Parisian drivers have been ignoring the restriction, with around 1,700 fines of between 22 and 75 euros handed out by authorities on Tuesday alone, according to The Local.

Pollution levels worsened on Wednesday despite a day of half traffic, and a parliamentary report has cast doubt on the efficacy of the measures.

Along with odd-numbered cars, hybrid or electric vehicles as well as those carrying three or more people will be allowed to roam the roads. Foreign and emergency vehicles will be unaffected.

As extra passengers piled into public transport Paris' train network was showing signs of strain.

The RER B commuter line suffered serious delays on Tuesday, thanks to a power problem, and n Wednesday, services were suspended at Gare du Nord station after a power cut.

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“There have been big problems” on the public transport network in Paris, said the city’s mayor Anne Hidalgo, according to Le Monde.

“It’s the region’s responsibility to sort it out. I urge the President of the region to dedicate himself to this task instead of always debating other issues”.

Ms Hidalgo introduced a measure this summer to ban cars built before 1997 from the capital's streets in a drive to reduce air pollution in Paris.

The older, dirtier vehicles are not allowed to drive in Paris between 8am and 8pm from Monday to Friday.