Drivers will need to pay to use 10% of Britain's road network if traffic congestion is to be curbed, a report by motoring experts warned yesterday.

Road charging will need to be imposed on the busiest motorways - including the M6 near Birmingham and the M25 - and in gridlocked urban centres by 2050, the RAC Foundation found.

The only alternative would be for a large investment in road building - predicted to be £750bn over the next 50 years. That looks unlikely. The government has pledged £180bn under its 10-year transport plan. But that covers all modes of transport including the stricken railways.

But it also discovered that attitudes to congestion charging - charging a fixed fee for entering sections of urban centre, or charging by mileage on busy routes - are shifting, with more than 70% of drivers accepting them as part of a package of a transport improvements.

The survey of 500 drivers, in the report Motoring Towards 2050, conducted in March this year, revealed that three-quarters would find road tolls acceptable if there were an equivalent reduction in tax disc fees. But 55% would not be willing to pay tolls to drive in city centres.

The findings have led the foundation to be "reluctantly... convinced that in the future charging will be required". But the foundation, the campaigning arm of the RAC, warned that charging would be acceptable only if it was coupled with other measures including road improvements, better traffic management, fuel duty reductions and enhanced public transport.

If investment were low, congestion charges would have to be higher. But the government must ensure "charging is part of a fair deal and must not rip-off motorists," the report said.

Commenting on the findings, Sir Christopher Foster, the RAC Foundation chairman, said: "The bad news is that congestion will get worse without radical action. We know what the radical action needs to be. The urgency is to get on with it.

"Unless we see a large increase in road building, road charging will be needed. Many may be surprised that a motoring group has opted for charging but our report shows that the alternatives are worse.

"Without charging we would see increased congestion and possibly unpalatable and probably ineffective efforts to curb car ownership."

However, the research does not represent government policy. The 10 year plan makes no decision on road user charging although the Transport Act gives local authorities the right to apply to impose congestion charges - set to be imposed in central London from February.