The Gwent Levels is a low-lying patchwork of wetlands, divided by drainage ditches first dug by the Romans, that is so full of life that conservationists compare its diversity to the Amazon rainforest.

But this quiet corner of south-east Wales may soon disappear under concrete if the Labour-led Welsh government gives the go-ahead for a new 14-mile stretch of motorway to be carved through the ancient levels.

Civil servants in Cardiff are currently examining a planning inspector’s report on the proposal, which is designed to ease congestion in and around Newport, and the Welsh assembly is expected to vote on the plans at the start of December.

“It’s a terrible idea, simple as that,” said Rob Waller, a Newport resident and volunteer at Magor Marsh, one of the four sites of special scientific interest that will be affected by the motorway. “I was just sitting there in the hay meadow. There was a buzzard circling overhead and the rustling of reeds behind me. It’s a beautiful spot. Why would anyone want to destroy this?”

Waller was also lucky enough to have seen the cranes that flew over the Bristol Channel from the Somerset Levels and nested on the Gwent version. “It was a very special moment to see them but the nesting site is right on the proposed route. I’m not sure they’ll nest again here if they blast a six-lane motorway through it.”

Residents such as Waller believe that the area would not be at risk if it was closer to Cardiff. “There is a view that the area isn’t very important. It’s a backwater, that little corner in the south-east that doesn’t matter very much.”

Ian Rapell, the chief executive of Gwent Wildlife Trust, speaks with passion about the otters, water voles – that have been successfully reintroduced – and spectacular wading birds that live on or visit the levels. But he is just as keen to talk about the less obvious plants and animals that inhabit the network of drainage ditches – or reens. These include the largest, and one of the rarest, water beetles, called the king diving beetle, as well as the nationally scarce rootless duckweed (Wolffia arrhiza), the smallest vascular plant on Earth.

Rapell describes the reens as a “rich soup of life”, with around 150 nationally notable or at-risk invertebrates, and compares the richness of the biodiversity of the area to that of the Amazon rainforest. “It’s special. It’s a very good example of how humans have interacted with nature in a positive way,” he said.

The Romans first drained the area to create a bread basket for their army ahead of a push into the Welsh mountains. Over the centuries since then, cattle and sheep have been farmed, the reens acting as a watery version of the hedgerow, both in terms of penning in livestock and creating linked wildlife paradises. The apple orchards once rivalled cider production in Herefordshire. Now the area is a magnet not just for wildlife such as the cranes but for cyclists and runners who relish the flat straight lines.

There is history everywhere. A line of willows on Magor Marsh is a reminder of a route that monks used to follow; a mark on the church wall of St Thomas the Apostle shows how high the flood waters rose in the 17th century.

Facebook Twitter Pinterest Campaigners Rob Waller and Rob Hepworth. Photograph: Gareth Phillips/The Guardian

“In other, better-known, areas that sort of history is cherished but not here for some reason,” said Rapell. “The government talks about the economic gain and saving time for car and lorry drivers. But saving 10 minutes is a high price to pay for the loss of all this.”

The cost of building the relief road has risen to more than £1.3bn, although critics predict it will eventually top £2bn. The Welsh government, which has the devolved power to push the scheme through, claims that the road would make doing business – and living – in south Wales more attractive. It argues that for every £1 spent, £2 will come back into the Welsh economy. It promises that its environmental mitigation measures will protect the levels.

The plan was debated – and largely supported – by MPs in Westminster last week. Stephen Crabb, the Tory MP for Preseli Pembrokeshire in south-west Wales, said the project had national and international significance, pointing out that it would help link Wales to Ireland in the west and continental Europe in the east. He said he had been told by hauliers that the route was becoming a “no-go area” because of the bottleneck around Newport created by the Brynglas tunnels where the motorway narrows. MPs also pointed out that congestion around the area would grow from next month when tolls on the Severn bridge are scrapped.

Rob Hepworth, chairman of the campaign group Calm (Campaign Against the Levels Motorway), said the congested stretch of M4 was itself built as a bypass. “Now they want to build a second bypass. If you have a dicky heart and need a bypass on a bypass, you should think about changing your lifestyle,” he said. “We need something that’s healthier, a more viable way of solving the problem.”

I like the wildlife, the butterflies, the insects. I don’t think they need a new motorway blasting through Llewelyn Pritchard, farmer

Hepworth says more effort should be put into creating more sustainable transport systems. The Welsh government counters that it is funding the delivery of a South Wales Metro and is committed to improving rail connectivity between the region and western England.

But critics such as Hepworth accuse the government of risking a breach of its own Wellbeing of Future Generations (Wales) Act 2015, which won praise from around the world, including from the UN, for seeking to ensure that long-term impacts are at the heart of all political decisions. If the motorway is given the go-ahead, legal action may be taken against it on this point.

The Future Generations commissioner for Wales, Sophie Howe, whose job is to make sure the act’s ambitious aims are fulfilled, has strongly criticised the relief road plan. She published a report setting out alternatives, and said: “The Welsh government’s obsession with addressing 21st-century transport issues with 20th-century solutions must not continue. The route fails to consider future trends and does not reflect the ambition or intention of the act.”

It is more basic than that for some residents who will be forced to move if the road plan goes ahead. Alan Ludlow, 78, explained that the plans for the new motorway had hung over him for two decades. His home is slated to be knocked down to make way for a transport yard. “It seems a shame to rip all this up. I don’t know where I’ll go,” he said.

Llewelyn Pritchard, 70, said his family had farmed on the levels for three or four generations. “I like the wildlife, the butterflies, the insects. I like to watch water voles and grass snakes. I don’t think they need a new motorway blasting through. I just do not know where we’ll all go.”