The five-month inquiry was due to start next month into the proposed 23km new section of motorway. But the Welsh Government has put back the start to March next year to give it time to assess the new growth data.

The Welsh Government pledged the delay would not affect the planned Spring 2018 construction start date for a Costain/Vinci joint venture which holds an ECI contract for the massive new section of motorway.

The UK Treasury has agreed to support the project by allowing the Welsh Government to borrow £500m towards the cost.

Cabinet Secretary for Economy and Infrastructure, Ken Skates said: “The UK Government has already expressed its desire to see work on the M4 project start as soon as possible, so it is hugely disappointing that the Department for Transport chose only to engage with us on these amendments to their traffic forecasting three days before they were introduced forcing this regrettable delay to the start date of the public inquiry.

“Assessments indicate that our proposed M4 Project is the only reasonable solution, but it is only right and proper that the proposals are scrutinised robustly, transparently and openly in an inquiry and that we test the project in light of the most up-to-date information available to ensure that we deliver the right choice for the people in Wales.

“It is essential that we, and others who wish to submit evidence to the public inquiry, now take time to fully review these new Department for Transport figures before the inquiry begins. 

“We’ve made a strong commitment to deliver a sustainable, long-term solution to the chronic traffic problems on this stretch of the M4 and I have been assured that, subject to inquiry approval, the M4 Corridor around Newport Project will still be completed in 2021.”

A Department for Transport spokesperson said: “We keep our traffic forecasts under review and made clear to Welsh officials at the start of this year we were going to update our projections.

”We are committed to working closely with the Welsh government and encourage them to deliver this project which will improve journeys for drivers.”

The new section of motorway is planned to be built between Junctions 23 and 29 south of Newport, crossing the Gwent Levels, River Usk Estuary, Newport Docks, the Llanwern Steelworks site and close to the Docks Way landfill site.

It will include a 440m main span cable-stayed bridge ranking as the fourth longest of its type in the UK.