Conservationists have criticised plans for a new road tunnel under the Thames which they say will destroy “irreplaceable” ancient woodland and harm wildlife.

National Highways says the Lower Thames Crossing is the UK’s most ambitious roads project for more than 30 years, almost doubling road capacity across the Thames east of London and easing congestion on the Dartford Crossing.

The roads agency said the new route would connect residents to jobs, boost the economy and create new public parks and replant 260 hectares of new woodland – six times more trees than lost.

But the Woodland Trust has criticised the plans, currently out for public consultation, which they say will destroy ancient woodlands, fell veteran trees, increase pollution and affect wildlife such as hazel dormice.

The woodland charity also raised concerns about a lack of transparency over the environmental impacts of the taxpayer-funded scheme, and warned it did not match the Government’s rhetoric on tackling the climate and nature crises.

The trust said woods threatened by the scheme form part of the Shorn and Ashenbank ancient woodland complex, including the charity’s own Ashenbank Wood.

The woods are home to rare wildlife, including woodpeckers, great crested newts and dormice, as well as important bug species and 300 types of fungi, the Woodland Trust said.

The charity’s assessment suggests several hectares of woodland would be lost, while it also warns that remaining ancient and veteran trees and woodland would be hit by nitrogen pollution from exhaust fumes.

Jack Taylor, lead campaigner, the Woodland Trust said: “The loss of irreplaceable habitats like these is unacceptable. We’re fighting both a nature and climate crisis, and destruction like this for a road scheme beggars belief.”

And he said: “National Highways (formerly Highways England) has not disclosed details of which environmental features will be affected and to what extent.

“Hiding these impacts from the public while consulting on the proposals is totally unacceptable.”

The increase in miles driven caused by the tunnel will push up carbon emissions and affect the landscape of the Kent Downs Area of Outstanding Natural Beauty and the Green Belt, National Highways’ own assessment shows.

The Woodland Trust is urging the public to respond to the consultation, calling on National Highways to give a clear appraisal of how much irreplaceable habitat could be damaged or destroyed, find ways to prevent the loss of ancient woodland and veteran trees, and avoid impacts on Ashenbank Wood.

Matt Palmer, executive director for the Lower Thames Crossing, said: “The Lower Thames Crossing will be the greenest road project ever built in the UK, and we are committed to not only reducing its impact but leaving a positive, lasting environmental legacy.

“We have avoided sensitive areas where possible and will be planting hundreds of thousands of new trees that will significantly outstrip the number of trees which will be lost when building the new road.

“Our plans also include exploring carbon neutral construction, creating a large community woodland at Hole Farm, two new public parks and an unprecedented programme of biodiversity enhancements that will provide bigger, better and more joined up habitats for wildlife and plant life.”