THE Welsh Government should accept its plans for an M4 relief road are doomed, and instead begin planning a South Wales Metro system, say Welsh Liberal Democrats.

With AMs due tomorrow to debate the Assembly's environment and sustainability committee's report into the M4 project, the party's shadow transport minister Eluned Parrott AM said it is time to look beyond this proposal.

Welsh Government consultation on the latest revival of the M4 relief road proposal has been criticised for a perceived failure to consider other options and for not addressing environmental concerns.

Ms Parrott said the proposal is based on a false premise, as the predicted effect a proposed South Wales Metro system would have in reducing vehicles on the region's roads, has been ignored.

Her party wants work on the South Wales Metro system - a key part of its budget agreement with the Welsh Government - to start immediately.

Labour's Newport East and Blaenau Gwent AMs John Griffiths and Alun Davies have criticised against the M4 plans and Ms Parrott said: “Rarely do you see Labour backbenchers stand up to Cabinet colleagues, but even they have spoken out on the M4.

"The chances of the Welsh Government ever getting a strong enough majority to force through its plans are slim to none.

“In budget negotiations, the Welsh Liberal Democrats agreed that no construction of the M4 relief road will start before the next Assembly elections and that a detailed Environmental Impact Study into the project will be commissioned.

"Therefore if the Welsh Government wants its preferred route for the M4 to be built, then it must survive a legal challenge, produce a convincing environmental assessment, win a majority at the next Assembly elections, convince its own AMs of its merits, then face a public inquiry. It seems unlikely their proposals could survive such a process."

Ms Parrott added that rather than "wasting a billion pounds on this dinosaur road" detailed planning should begin on the South Wales Metro system that "could take 40,000 cars off our roads every day."