Eluned Parrott AM has called on the Welsh Government to accept that its flawed proposals for the M4 are doomed.
The Welsh Government's consultation on the M4 has been widely criticised for failing to look at other options and for not properly taking into account environmental concerns.
This week in the National Assembly, AMs will debate the Environment Committee's report into the M4 the contents of which the Welsh Government failed to take into account before making its decision on the M4.
However, Eluned Parrott AM has accused Labour's M4 plans of being based on a 'false premise' due to the fact they do not take into account the effect the proposed South Wales Metro would have in reducing the number of drivers.
The Welsh Liberal Democrats have been campaigning for work on the South Wales Metro system to start immediately: something the party achieved in its budget agreement with the Welsh Government.
According to a Welsh Government commissioned report, the predicted usage figures for the core Metro area are around 40,000 daily journeys. This figure is not referenced in the Welsh Government's consultation on the M4.
Eluned Parrott, Welsh Liberal Democrat Shadow Transport Minister, said:
"The Welsh Labour Government is sticking its head in the sand over the M4. It is pretending all is fine and that it can plough on regardless. Yet we all know that's not the case.
"Rarely do you ever see Labour backbenchers stand up to their 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 Assembly Members of its merits, then face a public inquiry. It seems unlikely their proposals could survive such a process. It's time the Welsh Government looked again at its plans and start working with other parties to reach a proper solution.
"The Welsh Liberal Democrats also secured an agreement that commits the Welsh Labour Government to start detailed planning on the South Wales Metro system. To fit South Wales for the future we need to deliver a modern, clean and effective public transport system. If we get that right, we could take 40,000 cars off our roads everyday. We don't need to waste a billion pounds of public money on this dinosaur road."