The prime minister has promised to build a £1.3billion M4 relief road south of Newport in a bid to address congestion on the motorway.
Boris Johnson made the announcement at prime minister’s questions this week when he said he would unblock the Brynglas Tunnels with a new road.
He was responding to a question from Plaid Cymru MP Liz Saville Roberts, questioning the commitment of Secretary of State for Wales, Simon Hart, to supporting the Welsh economy.
Mr Johnson said: "I have not met anyone who is more bullish about the future of Wales than the Secretary of State for Wales.
“That is because this government is absolutely committed to levelling up across the whole of the UK, in Wales and everywhere with investment in infrastructure, education and in technology.
"We will do the things that the Welsh Labour Government, have failed to do, such as unblocking the Brynglas Tunnels and allowing that proper M4 bypass which has long been needed.
"We will provide the Vicks inhaler to the nostrils of the Welsh dragon and get Wales moving."
However, the decision is a devolved one and it is not clear how the UK Government could deliver the project without legislation rowing back the devolved settlement.
Last year First Minister Mark Drakeford rejected an independent inspector's report recommending the building of an M4 Relief Road on cost and environmental grounds.
A commission set up by Welsh Government, chaired by Lord Terry Burns, is now looking at other interventions to relieve congestion on the ageing stretch of motorway between Newport and Cardiff.
Mr Drakeford has consistently said there will be no U-turn on his decision.
Speaking in December Mr Drakeford said: "I’ve made my decision and it is an entirely devolved decision. The Prime Minister has no say in the M4 Relief Road whatsoever. It is not the way the system works.”
For news tailored to your local area, powered by In Your Area:If forced upon the Welsh Government he said: “Then you’re talking about very big changes indeed and a prime minister that would launch an assault on devolution... the way devolution works is the money comes to the Welsh Government and it is then for the National Assembly of Wales to decide how the money is spent, not for a Prime Minister in London.”
Mr Hart later acknowledged it was a devolved matter, but the Treasury stood ready to support the project with a specific borrowing facility.
He said: "The UK government has said it would provide the borrowing facilities, the ability to do that. But consistently Welsh Government have declined that offer and so we are at that rather stalemate period."
