'Destruction' warning over Newport M4 relief road plans
Ten of the UK's leading environmental charities have written an open letter to the Welsh Government objecting to current plans for an M4 relief road.
They said the £1bn road around Newport represented "ecological destruction on an unprecedented scale".
The so-called "black route" would cut through several protected Sites of Special Scientific Interest (SSSIs).
The Welsh Government said it was committed to the new road but a public inquiry would "scrutinise the options".
Signatories of the letter include the directors and chief executives of Wildlife Trusts Wales, Friends of the Earth Cymru, RSPB Cymru, the Campaign for the Protection of Rural Wales, Sustrans Cymru, the Campaign for Better Transport, the Woodland Trust, Buglife, Butterfly Conservation Wales and the Bat Conservation Trust.
They claim "the weight and strength of environmental evidence alone" should lead the government to withdraw the current plans, aimed at cutting traffic congestion.
"The habitat loss, combined with the landscape and visual impact on the Gwent Levels and its surroundings, would not only be brutal but irreversible," the letter states.
It argues the money to be invested in the black route should rather be spent on "public transport, active travel and better use of rail freight".
The government's decision whether to plough ahead with the scheme would also be "a true test" of the credibility of its new law, the Well-Being of Future Generations Act, the charities said.
"It will undermine key Welsh Government policy goals and legislative targets on environmental protection, modal shift, carbon reduction, air pollution and public health."
The proposals - first put forward 25 years ago - are for a 15-mile (24km) motorway and six-lane bridge over the River Usk to ease problems from the bottleneck at the Brynglas tunnels.
Speaking to BBC Radio Wales' Wales at Work programme, Economy and Infrastructure Secretary Ken Skates said the Welsh Government was committed to the road.
But he said he now wanted the discussion around the scheme to be "taken out of the hands of politicians".
"I'm keen that we should proceed with a public inquiry as soon as possible to be able to determine and to scrutinise the options that are available and also to explore whether there are any other options that are viable", he said.
Copies of the letter have also been sent to First Minister Carwyn Jones, Environment Secretary Lesley Griffiths and the Future Generations Commissioner Sophie Howe.
BBC Wales revealed last month that the statutory body in charge of the environment Natural Resources Wales had also submitted a 95 page objection to the plans as they stand.