The £1.4Bn scheme would have seen 23km of new dual three lane motorway built between junctions 23 and 29, on an alignment crossing the Gwent Levels, the River Usk estuary and Newport Docks.

But despite an inspector’s recommendation to proceed with the project following a public inquiry, the First Minister has now declined to make the draft Schemes and Orders which would have been needed for it to go ahead.

In a decision letter, he revealed that a meeting of the Welsh Cabinet in April concluded that the cost of the project was not acceptable in light of other demands and potential demands on the Welsh Government’s capital budget and uncertainty as to its financial position.

“Accordingly, the Welsh Government’s position is that it will not provide funding for the project,” he wrote.

He continued: “I recognise the inspector’s conclusions as to the advantages and disadvantages of the project. However, I attach greater weight than the inspector did to the adverse impacts that the project would have on the environment.”

In particular, he emphasised the “substantial adverse impact” that the relief road was expected to have on the landscape and wildlife of the Gwent Levels, which includes sites of special scientific interest.

In an address to the Welsh Assembly yesterday, the First Minister said there is a consensus he shares “that the issues of capacity, resilience and environment at the M4 corridor around Newport do have to be addressed, and that they will need a mixture of local and regional solutions”.

He said that a commission of transport experts will be appointed to review the evidence and make recommendations to the Welsh Government on alternative measures.

Welcoming the decision on the M4 relief road, Friends of the Earth Cymru director Haf Elgar said: “This devastating road would have ploughed through the unique, wildlife rich Gwent Levels, pumped more climate wrecking emissions into our atmosphere and ultimately caused even more congestion and air pollution.

“We now need substantial investment in sustainable transport around Newport and across Wales to help build a cleaner, safer future for us all.”

Business group CBI Wales’s director Ian Price described the M4 relief road’s cancellation as “a dark day for the Welsh economy”.

“Congestion and road pollution around Newport can only increase. Economic growth will be stifled, confidence in the region will weaken and the cost of an eventual relief road will rise,” he said.

(Photograph: Arup)