Speaking at the launch of a report outlining the state of the technology, Mr MacGregor said that the Government had taken no decisions on introducing it but warned that 'new ways of preventing road congestion in London from getting worse' had to be considered.
He emphasised that, at this stage, it was impossible to know what area of London would be covered, who would be exempted or what the costs of using London's roads would be.
He said that road pricing would need to await the development of technology and new legislation. The report says that the technology needed to cover a complicated area such as London is not yet available but will be developed in the next decade.
Although previous transport secretaries such as Lord Parkinson were strongly opposed to road pricing, Mr MacGregor, who has now been in the job for nearly a year, has recently become more enthusiastic about the potential benefits of road pricing and is planning visits to Norway and the United States to look at its development.
Although Treasury rules generally prevent it, he said that it might be possible to use revenues from road pricing to invest in roads or to improve public transport. Alternatively, the private sector could operate it.
Steven Norris, the Minister for Transport in London, was also supportive of the concept, saying: 'Implicit in the charging regime is the concept that at some point you are altering commuter habits.'
A number of types of charging are being considered. Charges could be linked to distance travelled or time spent in an area, or there could be higher charges at peak times with certain times or days being free.
The report, by Newcastle University, part of a pounds 3m research programme due to end next year, says a sophisticated system allowing drivers to be charged without stopping would be needed for London. Motorists who did not pay would have their number plates photographed.
The project director, Professor Peter Hills, explained that there had already been two generations of technology for in-car devices and it was the third, transponders, which allowed two-way communication between cars and roadside beacons, that could offer real opportunities to develop the concept.
No proper road pricing system exists in the world. Oslo and Trondheim in Norway, like the new Dartford Bridge, have toll booths where motorists can either pay in cash or drive through using simple pre-paid electronic devices. In Singapore drivers in the central area need a special disc but this is checked manually by police officers.
Professor Hills said that Singapore wanted a very sophisticated system to replace the present one, but the technology was not yet available.
Cambridge was one of five towns in Europe where different aspects of new technology were about to be tested. The scheme in Cambridge, involving a meter in the car linked to the mileometer, and a pre-paid 'smart' card would charge motorists only when their average speed was low, reflecting the presence of congestion.
Tony Kellett, technical director of Peek, one of the companies producing Trafficmaster, an in-car display that warns of congestion on motorways, said: 'I can envisage a time when all new cars will be fitted with transponders. The advantage is that they can be used for many different things, ranging from urban road pricing, to paying for car parking and motorway tolls.'
He said road pricing would eventually be accepted: 'People pay for static road space - car parking - quite happily. Why shouldn't they pay for moving along road space?'
The motoring and road organisations were against the idea, saying that public transport had to be improved. Edmund King, campaigns manager of the RAC, said: 'The Government could be backing a four-wheeled poll tax.' Paul Everitt, assistant director of the British Road Federation, said: 'Road pricing is years away. They should be investing in public transport.'
London Congestion Charging; Department of Transport; pounds 10.
Leading article, page 25