Environmental campaigners have raised concerns about what they say will be the negative effect on wildlife from a planned motorway relief road.
The area is the habitat of many rare species of creatures
The road in south Wales poses "the biggest single threat to wildlife", the Gwent Wildlife Trust has claimed.
If it goes ahead, it said damage will be caused to the Gwent Levels Sites of Special Scientific Interest (SSSIs).
The planned 15-mile three-lane motorway was announced in 2004 and is expected to cost at least £350m to build.
It has been designed to combat one of the most congested stretches of the M4.
The Welsh Assembly Government said it was part of its transport strategy for the area.
The proposed route crosses the Gwent Levels including several SSSIs and it is this that wildlife campaigners fear will be damaged.
Julian Branscombe from the Gwent Wildlife Trust, which held an open day on Friday to highlight the issue to around 150 visitors to the Magor Marsh site, said: "The proposed road will not provide relief, it will encourage more car use.
"It won't relive congestion, more cars will use the road and this all contributes to global warming.
Environmentalists fear the road will have a negative effect on the area
"There are alternatives to be had. We have got a wetland wonderland at the moment and the impact on the environment will be severe if the road goes ahead."
The Gwent Levels comprises an ancient farming landscape of grazing marshes and drainage ditches and is home to a variety of species of rare creatures.
Steven Rogers, also from the Gwent Wildlife Trust added: "Surely our politicians owe us more.
"As modern, well-educated people, they should have the wit to find more sensible, sustainable solutions to our transport problems."
According to the Welsh assembly government website: "great care will be taken to minimise the effects on the SSSIs by using previous industrial land where feasible".
Studies are being carried out about the proposed road.
Peter Fox, the Conservative candidate for the area in the forthcoming assembly elections, described the issue as "a balancing act".
"I'm extremely concerned about what the proposed road could do to the local environment but I'm also aware of the need to do something about the road system too," he said.
A Labour spokeswoman said: "Labour is committed to both improving transport links in Wales and protecting our environment and local wildlife.
"Data gathering for a full environmental impact assessment of the proposed relief road has been under way since May 2006. This assessment will identify measures to prevent, reduce and where possible offset any adverse effects on the local environment."
Plaid Cymru's candidate Trefor Puw said: "We have serious environmental concerns about the M4 relief road and its effect on the Gwent levels.
"Plaid believes that the money could be better spent improving public transport and the Gwent transport infrastructure."
Ed Townsend, standing for the Liberal Democrats, said: "When you start looking at the need for building roads, there are balances to be struck.
"We have to balance the needs of the natural environment and the people who live in the urban environment.
"On balance, I come down on the side of the road. The place is grinding to a halt.
"In an ideal world better public transport is a far more attractive option but until that time, we have to do something about the motorway."