AMs have claimed the cost of an M4 relief road could rise by an extra half a billion pounds - and urged ministers to take plans “back to the drawing board”.

Conservative transport spokesman Byron Davies called for a “full and swift review” into the planned road to the south of Newport, saying the rejected ‘blue route’ should be looked at again.

In July last year the Welsh Government revealed they had opted for the £1bn 'black route', one of three proposed options for the new road, despite opposition from cross-party AMs and environmental organisations.

The route plans to take the motorway from the existing junctions 23 at Magor to 29 at Castleton, going through part of the Newport docks.

But Byron Davies said at the Senedd yesterday: “Quite honestly the minister has charged into coming out with the black route without, we think, giving proper consideration to the blue route. We’ve not had an opportunity to really scrutinise it in the chamber and I would like that opportunity.”

Paul Davies, AM for Pembrokeshire, also raised concerns. He said: “They are forecasting it will cost £1bn now. We know with every road project we’ve seen it goes over budget. Perhaps it will even cost £1.5bn.”

Mr Davies admitted there could be issues with pollution for local people if the blue road using upgrades to the SDR as its basis was accepted, but said there would be environmental considerations no matter what option is taken forward.

Byron Davies said: “This is something which I have spoken about at length with the various transport companies, the freight transport people, and quite frankly all they are interested in is having an alternative route, whether it is blue, pink, red, green or whatever. This is the workhouse of Wales here in South Wales and they need it as soon as possible.

“We think the black route is an awful lot of money. There are other options to be considered. We do not think other options have been given sufficient consideration. We would like them to go back to the drawing board on it.

A Welsh Government spokesman said: “The Tories are putting political point-scoring ahead of economic development. When the Tory Chancellor, George Osborne, last came to Wales he said the M4 wasn’t progressing quickly enough – now the Welsh Tories want to kick things into the long grass. You can’t have it both ways. The Welsh public and Welsh businesses will want to know who is really speaking for the Tories on this matter.

“The Welsh Government believes that the proposed changes are hugely important to the economic prosperity of Wales as a whole.”