Government plans for another tolled motorway could backfire, the AA has warned.

It is thought that Chancellor George Osborne will announce a new 14-mile M4 relief road near Newport in South Wales on which drivers might pay a toll. This would be the UK's second tolled motorway following the 27-mile M6 Toll around Birmingham.

The M4 plan is believed to be part of a major road announcement by Mr Osborne in his spending review in June.

AA president Edmund King said that M4 improvements were welcome but introducing tolls could deter motorists. He went on: "We have seen with the M6 Toll that drivers can be turned off by the imposition of charges.

"There is already a toll to get into Wales on the Severn Crossing but no toll to get out. There is no doubt that the M4 around Newport needs improving. At the moment if there is a bad accident and the road is closed there is no real alternative for drivers. But if the road is tolled it reduced its economic effectiveness as a lot of people will be put off using it and will simply avoid it."

An M4 relief road has been proposed before but was dropped on cost grounds. Other schemes Mr Osborne is looking at include improvements to the A303 - the road that passes Stonehenge in Wiltshire. Other projects may include making all the section of the A1 north of Newcastle dual carriageway and improvements to the A47 in Norfolk.

Matt Hemsley, Cymru media and policy adviser for sustainable transport organisation Sustrans, said: "Investing in new roads is not the answer to congestion, which is set to cost the Welsh Government £1.1 billion a year. The new M4 relief road is poor value for money - costing taxpayers more than £55 million each year and following the recent announcement of £8 million in cuts to our bus service.

"The Welsh Government acknowledges that 40% of journeys on this stretch of the M4 are local trips of under 20 miles. A more cost-effective way to reduce congestion would be to invest in shifting these journeys to public transport. By making it easier for people to walk, cycle and catch public transport, we can ease congestion, improve our health and save the economy billions."

A Welsh Government spokesperson said: "We do not comment on speculation but, as previously stated, we have been pressing the UK Government for access to borrowing powers in order to finance infrastructure investment. Those talks are continuing. Meanwhile, we continue to consider the large volume of responses to our consultation on M4 corridor enhancements from Magor to Castleton."