Plans to pedestrianise a stretch of the Left Bank in Paris could spell traffic chaos for the French capital, regularly ranked among Europe's most congested cities, a report said on Thursday.

France's new left-wing government last month gave the go-ahead to ban cars from a 2.3-kilometre (1.4-mile) stretch along the Seine between the Alma bridge and the Musee d'Orsay.

The previous administration had vetoed the plan.

The Left Bank will be partially pedestrianised from spring 2013 and work has already begun to transform a section of the Right Bank.

There, they are cutting the number of car lanes and creating a 1.5-kilometre (one-mile) promenade stretching east from city hall opposite the Ile Saint-Louis

The city authorities are spearheading the project.

But a confidential study carried out for city hall and obtained by Le Parisien newspaper warns that during the rush hour between 800 and 1,200 cars every hour will somehow have to find new routes across the city.

Both sides of the Seine, home to many of Paris's top attractions including the Eiffel Tower, Notre Dame Cathedral and the Louvre, are currently used as an expressway allowing cars to cut across the capital.

Paris was ranked Europe's fifth most congested city in a survey released this month by the Tom Tom satellite navigation firm. Two years ago a separate study ranked it the most congested.

To avoid gridlock from the river-bank scheme, the 450-page leaked report says large numbers of Parisians and suburban residents will need to switch to public transport.

Paris mayor Bertrand Delanoe has led a decade-long policy of reclaiming space for cyclists and pedestrians.

It says in the report it is counting on a 10-percent reduction in inner-city car traffic in the next five years.

Long stretches of the banks of the Seine, which are classed as a UNESCO World Heritage Site, have been pedestrianised for a month every summer since 2002, for the hugely popular Paris Plages riverside beach bonanza.

Both the future Left and Right Bank pedestrian zones would host riverside bars and restaurants, parks and children's play areas in the spirit of the summertime Paris Plages event.