The government is carrying out trials of technology that will charge drivers for every journey they make. The Department for Transport admits that the £23m trials, which are being kept low key, could pave the way for the introduction of national road-user charging.

Cameras that read numberplates are being used in conjunction with satellite and microwave technology to track the journeys of 450 volunteer drivers in South Yorkshire.

The experiment appears to confirm suspicions that the government is further advanced with plans for road charging than it has been willing to declare. The trials, which started in February and have received no publicity, are codenamed Directs (Demonstration of Interoperable Road-User End-to-End Charging and Telematics Systems).

The revelation has provoked an angry reaction from some motoring organisations and opposition politicians, who accused ministers of a hidden agenda to put a mileage tax on motorists.

Seven 25ft steel gantries have been erected on the M621 between junctions 5 and 6; on the A639 link between the M1 and the M621; the A61 between Leeds and Wakefield; and on local roads south of Leeds. Special cameras and microwave transmitters will monitor the journeys of the 450 motorists for the next 10-15 months. They will be sent monthly bills (though they will not pay anything at this stage). Devices the size of an audio cassette have been installed behind the dashboards of their vehicles to receive microwave signals from the roadside beacons. The registration numbers of the vehicles will also be recorded as they pass by.

The trials are viewed as the first step towards a system that could see motorists paying up to £1.40 a mile to use busy roads at peak times. Alistair Darling, the transport secretary, has said road charging could be in place by 2014. But last week a report by the Commons transport select committee urged the government to press on with plans for road pricing.

“We cannot afford to wait 10-15 years for the technology for a national system to arrive before testing the effectiveness of road pricing,” said the report.

The number of vehicles on British roads is predicted to increase by 30% by 2015, and Darling has warned of an American-style gridlock without “radical” solutions such as road pricing.

The trials of the new road-charging technology come on the back of research by the Transport Research Laboratory (TRL) at its facilities in Crowthorne, Berkshire, during 2004. The TRL looked at different systems for monitoring and charging motorists for their journeys.

Three charging systems are now being tested on the roads in South Yorkshire by the Department for Transport. The first harnesses global positioning system technology, which uses satellites to pinpoint vehicles, the second uses microwaves transmitted from beacons on the gantries to the audio-cassette-sized receivers, and the final system works in the same way as the London congestion charge, using a system of automatic numberplate recognition to track vehicle movements.

Department for Transport officials argue that if a system were to be adopted across the country, there would be no need for drivers to stop at toll booths or remember to make daily payments by phone or the internet, as is the case in London. Instead, bills would be sent automatically at the end of each month.

A government spokesman said there were no immediate plans to introduce road-user charging but it was under consideration. “There is a lot more work to do before we can decide whether road pricing is feasible,” the spokesman said. “It is not current government policy to introduce a scheme but to consider one is.”

There are, however, increasing signs that the government views a national scheme as the only viable way to reduce congestion and meet its targets for reducing carbon dioxide emissions.

The £2 billion set aside to help local authorities invest in London-style congestion charging has so far failed to persuade a single council to introduce a scheme. In a referendum in February, Edinburgh residents voted against plans to charge motorists £2 for entering the city centre. Other towns and cities, including Bristol and Southampton, have put plans on hold..

Under a national scheme Britain’s 30m motorists would be charged between 1p and £1.40 per mile depending on the type of road and time of day, according to a feasibility study compiled on behalf of the government by an expert group including motoring representatives, civil servants and environmentalists and published in July 2004. The study suggested road charging could halve congestion during peak times.

The government plans to introduce road pricing for lorries in 2007-8, with the promise of fuel duty discounts to offset the new charges. A similar system launched in Germany earlier this year has suffered technological problems.

The Conservatives have said road-user charging would be acceptable only if it were coupled with reductions in fuel duty and road tax for all road users. Tim Yeo, the shadow transport secretary, said: “This taxpayer-funded secret experiment looks as though the government is paving the way for another stealth tax which will hit motorists.

“If you ask motorists to pay more charges then you should provide either a cut in other motoring taxes or an improvement in the road network.”

The RAC Foundation is broadly in favour of road pricing — “provided it is fair and monitored by an independent body”.

The Association of British Drivers predicted drivers would refuse to pay. “Road-user charging is a white elephant,” said the organisation’s spokesman Nigel Humphries. “It’s just another excuse not to spend any money on transport. People already use the roads in the most efficient way they can. They don’t sit in jams unless they need to. People already pay more to use the railways at peak times and it doesn’t work. Trains are still packed at rush hour and empty during the day.”