Motoring groups expressed fears today that drivers could face increased toll charges under Government plans to sell off the Dartford Crossing on the M25 - Europe's busiest motorway.
The AA said Whitehall had already "reneged" on an earlier promise that tolls would cease when the crossing was paid for.
Although the crossing was paid for by 2002, the Government introduced new legislation which enabled the fee to become a charge and not a toll.
The switch from tolling to charging came in April 2003, with the Department for Transport saying that money raised would "only be spent on transport improvements which will affect motorists".
The current fee is £1.50 for cars between 6am and 10pm, with the largest lorries paying £3.70.
AA president Edmund King said today: "Motorists will not care who owns the Dartford Crossing but they do care about the level of tolls. Government has already reneged on the original promise that once the crossing was paid for and a maintenance fund set up, then tolls would be dropped.
"We have recently seen tolls increase to £1.50. If the crossing is sold there must be watertight clauses that govern toll increases.
"We cannot have the private sector dictating the level of toll that motorists should pay on the most important motorway in the UK that motorists have already paid for many times over in tolls and taxation."
Situated on the Essex-Kent border section of the M25, the crossing consists of two dual-lane northbound road tunnels and the towering four-lane southbound Queen Elizabeth II Bridge, which opened in 1991.
More than 50 million vehicles use the crossing each year - an average of around 145,000 vehicles a day. The record day for the crossing was July 23, 2004 when nearly 182,000 vehicles used it.
Since last month, the crossing has been operated on behalf of the Highways Agency by Connect Plus, a consortium whose companies include engineering giants Atkins and Balfour Beatty.