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Motorists are set to face increasing motorway tariffs for years to come, it has been claimed.

Broadcaster France 2 has said that a secret agreement between the government and the motorway companies means that drivers using the country's toll roads will face annual price increases until 2023.

The agreement was reached in April 2015, France 2 said, after motorway companies had threatened to take the government to court over its 2014 decision to put a freeze tolls in 2015.

In it, companies pledged to finance €3.2billion in maintenance and improvement works over 10 years, in return for the right to increase tariffs between 2019 and 2023. France 2 estimates the additional cost to motorists will be €500million - an amount that would guarantee the financial stability of autoroute operators.

Ségolène Royal, who was environment minister at the time has denied her reported involvement in the deal.

Meanwhile, France 2 said that independent supervisory authority Arafer has voiced concern over the cost of the works, saying that it has been overvalued, and includes works that had already been budgeted for.