Parisians can use public transport for free for the second day running due to a spike in air pollution and some cars have been barred from the roads.

The city is suffering its worst and most prolonged winter pollution for at least 10 years, the Airparif agency which measures the levels said on Wednesday.

Authorities have said only drivers with odd-numbered registration plates can drive in the capital region on Wednesday. Drivers of even-numbered cars were given the same opportunity on Tuesday, but could now be fined up to €35 if they are caught behind the wheel. More than 1,700 motorists were fined for violations on Tuesday.

Paris mayor Anne Hidalgo said images of smog blanketing the capital were proof of the need to reduce vehicle use in the city centre.

The air pollution peak is due to the combination of emissions from vehicles and from domestic wood fires as well as near windless conditions which means pollutants have not been dispersed, the Airparif agency said.

“This is a record period (of pollution) for the last 10 years,” Karine Leger of AirParif told AFP by telephone.

For more than a week, Airparif has published readings of PM10 at more than 80 microgrammes per cubic metre of air particles, triggering the pollution alert.

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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 was still partly out of action on Wednesday morning. On Wednesday, services were suspended at Gare du Nord station after a power cut.

The Paris prefect, Michel Cadot, said the ban would be extended to a third day if required. “As long as the meteorological situation remains the same and emissions remain the same, we will not see the end of the pollution spike,” he said.