Image caption Plans for an M4 relief road were first announced in 2004

Plans to build an M4 relief road in south Wales are being backed by the Treasury, the BBC has been told.

It follows press reports that Chancellor George Osborne will announce plans to support a new toll motorway in June's comprehensive spending review.

However, the Welsh government said it does not plan to impose tolls on Welsh roads and called the idea "unworkable".

A Westminster source said agreement in principle had been reached but a deal on funding was not finalised.

The Welsh government dropped plans for an M4 relief road around Newport in 2009 after the cost rose to £1bn.

Talks have been continuing between the UK and Welsh governments about ways to fund improvements to the M4 in south Wales, which business leaders say are vital for the economy.

It's worth pointing out though that the M6 Toll in the Midlands... hasn't ever made any money. People won't pay it Mickey Clarke, Financial journalist

Reports in The Times and Independent say that funding for a relief motorway will be guaranteed by the Treasury to allow the Welsh government to build the road and then to repay the loan at least in part by raising tolls.

A Downing Street spokesman said: "Any decisions on future schemes would be taken following the spending review.

"Tolling the M4 in Wales would be a matter for the Welsh Assembly."

A Welsh government source said: "A toll road idea has not formed any part of the inter-governmental talks that have taken place between the Welsh and UK governments.

"The tolling of Welsh roads is entirely a matter for the Welsh government and we have no plans to introduce tolls on any Welsh road.

"It's an unworkable idea and given the Scots are not being forced to put tolls on their new Forth Bridge, it would be unfair for Wales to be expected to re-coup the money in this way, to fund road improvements here.

"Wales is not a second-class Scotland.

"As part of the ongoing inter-governmental talks, the future of toll income from the Severn Crossings after the current concession ends, is also being discussed."

The Welsh government said it was "following due process" on the issue of easing the congestion on the M4 around Newport, and that it was continuing to press the UK government for access to borrowing powers in order to finance infrastructure investment.

The UK's only toll motorway is the 27-mile (43km) M6 Toll north of Birmingham which opened in 2003.

Employers' group the CBI has called a relief road to ease congestion on the M4 near Newport a "clear priority".

Media playback is unsupported on your device Media caption Welsh affairs editor Vaughan Roderick says some form of borrowing may be needed to fund the project

Business leaders say traffic jams, particularly after crashes which close the motorway, are bad for the economy of south Wales.

Plans were announced in 2004 for a 14-mile road to ease congestion on the M4 around Newport, between junctions 23 and 29.

The scheme was estimated to cost at least £350m and was due to open in 2013, with the possibility that it would be partly financed by tolls.

When the scheme was scrapped in 2009, its likely cost had risen to £1bn.

Ieuan Wyn Jones, then deputy first minister and transport minister, said at the time that putting tolls on the "unaffordable" proposed new road would have reduced its economic effectiveness.

Financial journalist Mickey Clarke told BBC Radio Wales: "It's worth pointing out though that the M6 Toll in the Midlands... hasn't ever made any money. People won't pay it. That's the trouble."

These works have been in the pipeline for many years and should be completed before we look again at a very expensive toll road Jonathan Edwards MP, Plaid Cymru

Nick Payne, regional director of the Road Haulage Association, welcomed the idea of the proposed new motorway going ahead but was against the prospect of tolls.

He said: "The M6 Toll road hasn't been massively successful as far as the transport industry is concerned. I don't know whether or not the haulage industry in Wales can afford to be paying yet another toll after paying so much money to get across the [Severn] bridge every day."

Matt Hemsley, spokesman for the transport charity Sustrans Cymru, criticised the M4 relief road as "poor value for money", saying investment in public transport would do more to ease congestion.

"The Welsh government acknowledges that 40% of journeys on this stretch of the M4 are local trips of under 20 miles," he said.

'Save billions'

"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."

Media playback is unsupported on your device Media caption Nick Payne from the Road Haulage Association and Matt Hemsley from Sustrans give their reaction

Environmental campaigners have renewed their opposition to an M4 relief road amid fears of damage to the Gwent Levels Sites of Special Scientific Interest (SSSIs).

Tom Clarke, chief executive officer of the Gwent Wildlife Trust, said: "This would mean years of construction and pollution for the residents of Magor, Newport and Cardiff, and destruction of the nationally important Gwent Levels - and then asking them to pay for the privilege."

In February 2012 the Welsh government announced plans to turn an old access road through Llanwern steelworks into a £13m dual carriageway to link south Newport with the M4, claiming it would help relieve congestion.

Plaid Cymru economy and transport spokesperson Jonathan Edwards said he was sceptical about the proposals.

He said: "People want to lessen the impact of the tolls on those coming into south Wales, not increase it.

"These works have been in the pipeline for many years and should be completed before we look again at a very expensive toll road."