Prequalification for the massive motorway job started this summer on the early contractor involvement scheme.

Five firms are expected to make the shortlist for the job with the four-year construction programme scheduled to start in Spring 2018.

The 14-mile motorway between Junctions 23 and 29 will include a new crossing of the River Usk and represents the largest capital investment programme in Wales since devolution.

But environmentalists believe plans to build through one of Wales’ most important wildlife habitats may be unlawful.

Friends of the Earth Cymru has now launched a judicial review of the M4 plans after claiming the Government refused to reconsider the decision to build through the highly protected Gwent Levels wetlands.

Director of Friends of the Earth Cymru, Gareth Clubb said: “This motorway would cause massive damage to important and protected wildlife.

“We think the Welsh Government has acted unlawfully, and there are better transport options they’ve not properly considered. We’re now asking a judge to decide whether the government is breaking the law.

“People from all walks of life have joined us in supporting alternatives, from business groups to political parties.

“Whether it’s because of the huge expense, the better transport alternatives, or the environmental damage, many thousands of people in Wales are against this unnecessary and unaffordable road.”