MPs are considering an investigation into Britain's only privately-owned motorway after the cost of driving on it rocketed by 50 per cent in just four years.

The latest price hike comes after 500,000 motorists stopped using the M6 toll road in just three months.

Motoring groups said the inflation-busting rises called into question the Government's controversial road pricing strategy.

The 27-mile M6 toll road, which runs to the north east of Birmingham, was opened in December 2003 to alleviate heavy congestion along the M6 through Birmingham.

However, since then Midland Expressway, the private company which built the road and runs the tolls, has increased the charge to car drivers from £3 to £4.50 each.

The most recent increase, by 50p to £4.50 a car, was imposed on New Year's Day. For lorries and vans the toll increased by £1 to £9 per vehicle.

The price rise is a further blow for hard-pressed motorists already coming to terms with petrol costing over £1 a litre and increased charges for driving more polluting vehicles.

Motoring groups now fear Midland Expressway, which is laden by debt and lost £80,000 a day last year, could push the price through the £5 a car barrier.

Although overall more than 17 million drivers are using the toll road every year, the number of vehicles using the road in the three months to last September - the most recent figures available - fell by more than 500,000.

Midland Expressway blamed the fall on the removal of roadworks from a nearby stretch of the M6.

However the average rate of drivers using the road - 48,929 vehicles a day - is understood to be below experts' forecasts for the traffic levels before the road was built.

Gwynneth Dunwoody, the chairman of the House of Commons transport select committee, said MPs would look at travel increases over the holiday period, including the toll hikes.

"The relationship between private industry and transport is a fraught one. I have no doubt we will be considering this," she said.

Motoring groups were outraged by the latest price rise. Some say there is there is evidence that lorries are baulking at the high charges and using the free M6 instead, despite the traffic.

Midland Expressway confirmed just 2,000 lorries use the M6 toll road every day.

Sheila Rainger, the acting director of the RAC Foundation, said: "It is a big increase. Perhaps we will reach the limit at £5. But the road is owned by a private enterprise. There is no legislation about what they can or cannot charge."

If the toll became too much "people could vote with their wheels", she said.

Edmund King, the president of the AA, said there should be a review of the toll charges. "It is no surprise that the motorists don't trust the Government to be fair," he said.

The toll level has been a source of controversy since a director of the Australian company which owns Midland Expressway boasted in 2003 that he could "put up the tolls by whatever we like".

Dennis Eagar, a director of Macquarie Infrastructure Group, said: "If drivers don't complain about it being too high, then we won't have done our job."

He resigned after the remarks. There was further controversy in 2006 when Macquarie refinanced the entire project and cashed in a profit of almost £400 million just three years after it opened.

Thomas Fanning, chief executive of Midland Expressway, said last night: "Increasing a toll is not a decision we take lightly.

"We believe the new price tariffs remain excellent value for money for the convenience of a reliable, stress-free journey as compared to the congestion of the M6.

"We are very pleased with the overall growth of traffic on the M6 Toll and this is very much in line with the national average for growth on a motorway."

Asked if the toll would go through £5 per car next January, Mr Fanning said: "The current prices will remain in place throughout 2008 and will be reviewed in line with operational need."

He said the recent fall in traffic was "very much expected" because of roadworks on the M6 past Birmingham.

Regarding the company's losses, the spokesman added: "The M6 Toll was a £900 million investment which was made with a very long-term perspective under a 53-year concession.

"It is not unusual for a toll road at this stage in its maturity to experience a loss as the large costs of construction are taken into account over the life of the road. It has no bearing on toll decisions."

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