Government plans to force drivers to carry satellite-linked “black boxes” in their cars as part of a national pay-as-you-drive scheme are unlikely ever to be introduced because ministers fear a public backlash on privacy grounds.

Instead, the government is planning to introduce a system of “car tagging” which will be less intrusive and charge drivers only on a limited number of toll roads. When a vehicle passes a roadside beacon a tag installed in the car would trigger a charge for the use of that section of road.

Recent reports have suggested that Alistair Darling, the transport secretary, is planning to “tax” drivers on a per mile basis by tracking cars with GPS (global positioning system) technology. The government is likely to back away from these proposals nearer the time road charging is introduced, according to Department for Transport (DfT) sources.

The DfT has appointed an American contractor to head trials of its favoured system of car tagging. Kellogg Brown & Root (KBR), part of the Halliburton group, based in Houston, Texas, is in charge of a £23m project codenamed Directs (Demonstration of Interoperable Road-User End-to-End Charging and Telematics Systems).

The choice of KBR as the head contractor gives a strong indication that the government prefers the American-style charging scheme. Of the 550 vehicles taking part in the British trials 500 have been fitted with electronic tagging devices similar to those which have been proposed for a huge network of tolled freeways in Texas. Only 50 vehicles are using GPS technology.

Devices the size of audio cassettes have been installed on the dashboards of the cars of the 500 volunteers in Yorkshire. Seven pairs of 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.

The gantries also carry cameras that record numberplates. The results will be monitored from KBR’s UK headquarters in Leatherhead, Surrey, with plans to issue dummy bills and penalty notices later in the year.

Transport for London (TfL), the mayor’s transport arm, plans to carry out similar trials this summer using the same tagging technology. It hopes to install the technology in the current congestion charging zone from 2009. The tag being tested by TfL could also be used like a mobile-phone SIM card, storing cash credits which could be deducted automatically when cars pass a charge beacon.

In Texas — where KBR was involved in proposals for tolled freeways — there have been widespread protests at plans to introduce car tagging on the grounds that the state is taxing people to use roads the public has already paid to build.

Motorists have been told that under the scheme they will have to pay up to one dollar per mile in electronic road tolls, a similar concept to the British government’s proposals for charges of up to £1.34 per mile. Protest groups have united under the banner of the Texas Toll Party, which has rallied popular support among thousands of motorists.

Opposition is gathering such momentum in Texas — President George Bush’s home state — that the authorities may be forced to think again. The objections raised are almost identical to those levelled against Britain’s road charging plans.

“We already have a gas tax; we don’t need another tax,” said Sal Costello, founder of the Texas Toll Party. “They (the Texan governors) claim because of congestion and because there is not enough money coming from the gas (petrol) tax they need to charge us tolls as well.

But tolling is a sloppy tax. Most of the money goes into collecting the money and it is a new way of taxing people for something they’ve already paid for.

“This is an experiment for the whole country and all the other states are watching. It’s happening throughout Texas, and other states — Georgia, Michigan, California and others — are talking about it. Our fight here can make a difference.”

KBR has been at the centre of controversy in the US for other reasons. Brown & Root, which merged with MW Kellogg in 1998, was accused of “buying” political influence, and KBR has been investigated for overcharging the American military.

During the Iraq conflict in 2003 KBR allegedly purchased petrol in Kuwait and sold it at a profit to the US armed forces. The American defence department claimed the firm had overbilled by $61m (£33.5m). The company also allegedly overcharged for meals it provided in Iraq to US soldiers. It agreed to pay back $27.4m (£15m) to settle the issue.

Road pricing only recently hit the headlines in Britain but the government has been quietly planning it for years. In 2001 the DfT hired Fareway Alliance for the first phase of the Directs project and in 2002 drew up detailed plans for the installation of the pilot road charging scheme. The trials were scheduled originally to begin in 2004. Fareway Alliance is a consortium led by KBR with WS Atkins, a design consultancy, and Thales Translink, which provides electronic services.

In April the government hired Derek Turner, the man credited with designing and implementing congestion charging on behalf of Ken Livingstone, the mayor of London, to head up the Highways Agency, the body responsible for motorways and trunk roads in Britain.

Asked whether his appointment as national traffic director was linked to the government’s plans to bring in a tagging system for road charging, Turner said: “As a civil servant I am unable to comment.”

A spokesman for KBR said claims it had overcharged the American government for fuel were “misinformed”.

“KBR’s reputation is that of a company that continues to deliver for its government and military customers despite the challenges it faces while undertaking difficult tasks under some of the most dangerous and hostile conditions anywhere in the world,” the spokesman said.