Congestion charging is inevitable to prevent roads like the M25 motorway around London from becoming gridlocked, according to a leading government road transport adviser.

David Hardcastle's team of consultants have made a preliminary recommendation that road pricing should be introduced if the M25 is widened at least in the south east and preferably throughout the country.

They say if extra road space is created free of charge to drivers, it will fill up again and make congestion even worse.

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Mr Hardcastle will set out a timetable.

He will tell the Government's Programme of Multi-Modal Studies conference in London that ministers must commit to a pricing scheme within the next two years to keep pace with the road widening plans.

Postponing the decision on charging could plunge the south east into gridlock and cause grave damage to the national economy, he says.

Any new charges are vociferously resisted by the Association of British Drivers.

The government says a decision will be made after the consultants' final report.