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Public consultation on building an M4 relief road south of Newport will begin in September, the Welsh government has announced.

Transport Minister Edwina Hart said tackling congestion on the motorway was vital for the Welsh economy.

Earlier on Wednesday, the chancellor said he would shortly give details of "impressive" M4 relief road plans.

But it has not yet been confirmed how the project, costing up to £1bn, would be paid for.

Announcing his spending review for 2015/16 to MPs in the Commons, George Osborne said UK ministers would announce news on the relief road soon when responding to the Silk commission on devolved taxation powers.

But, in a written statement, Mrs Hart said: "As a result of ongoing discussions with the UK government there has been a significant change in the assessment of the affordability of a major enhancement of the M4.

"Building on the extensive development and consultation work undertaken on M4 Corridor Enhancement Measures (CEM), we will be consulting formally over the summer with Natural Resources Wales, in order to go out to public consultation this September with a finalised draft plan and Strategic Environmental Assessment (SEA) report.

"If implemented, the draft plan would lead to a motorway being built south of Newport."

Mrs Hart said that "addressing the capacity and resilience issues on the M4 around Newport is the top transport challenge that we face in ensuring that Wales has an effective economic infrastructure which improves our competitiveness and access to jobs and services".

Plans to ease congestion on the M4 in south Wales by building a relief road around the city were unveiled by the Welsh government in 2004.

Five years later the then Labour-Plaid coalition in Cardiff Bay shelved the idea when the estimated cost rose to £1bn.

Speculation in April than an M4 relief road might be financed by tolls has been dismissed.