Several nature groups are calling for an urgent review of updated plans for Britain's biggest road tunnel – including a full disclosure of its "cost and climate" impacts.

National Highways last week resubmitted its bid for the £8.2 billion Lower Thames Crossing road scheme connecting Kent and Essex.

The huge project, which consists of a 14-mile bored tunnel under the Thames, promises to relieve the substantial pressure on the county's road network and the oft-gridlocked Dartford Crossing,

Initial plans were pulled at the 11th hour in November 2020 after it emerged the Planning Inspectorate was poised to reject them.

It led to the government-owned company re-consulting on the plans which have since been redesigned alongside a commitment to plant 1 million trees.

But nearly two years on "serious concerns" remain over the controversial scheme and its eco-credentials ahead of an upcoming decision by planners to accept the bid for inspection.

National Highways says it recognises the need to build the crossing "in the right way", but adds it is doing all it can to minimise environmental impacts on local communities who will have a further chance to "challenge and scrutinise" all aspects of the proposals.

However, a coalition of conservation groups have now written to the Transport Secretary, Mark Harper, calling for an immediate review of plans.

The scheme has been discussed in different forms for a number years, but campaigners argue it should be scrapped on the basis of "cost and climate impacts".

Various groups – including the Kent Wildlife Trust, Buglife, and CPRE Kent – claim the road scheme would destroy irreplaceable ancient woodland, veteran trees and other habitats.

These impacts would likely make this "England’s most damaging road scheme for a generation", it adds, in stark contrast to claims by National Highways that it will be the "greenest road ever built in the UK".

Conservation organisations also argue the public have been left to guess the true scale of the scheme’s "huge carbon footprint".

It comes amid "serious concerns" over the level of detail provided through the various consultations and a lack of of "meaningful public engagement".

“At a time when Kent is increasingly facing the pressure of development, to proceed without the full impact of this project being made public is utterly unacceptable."

The coalition is calling on the Transport Secretary to commit to an immediate review of the proposals in light of "ongoing flaws".

Paul Hadaway, director of conservation for Kent Wildlife Trust said: “Ploughing ahead with the proposed LTC will result in the loss of irreplaceable habitats, including ancient woodlands and wetlands which are locking up significant levels of carbon, going against the government’s commitments to the environment.

“At a time when Kent is increasingly facing the pressure of development, to proceed without the full impact of this project being made public is utterly unacceptable.

"It shows a disregard for the important role that the impacted habitats play in protecting Kent and its population from the worst impacts of climate change.”

Hilary Newport, Director of CPRE Kent, added: “CPRE Kent has been frustrated by the lack of key information provided every stage of the consultation process.

"The last statutory consultation was back in 2018 and since then there have been multiple design changes.

"It’s not reasonable to expect a project of this scale to go to examination without full detail of its impacts.”

It comes after an application was submitted to the Information Commissioner's Office (ICO) by Thurrock council, urging the highways company to release its outline business case (OBC) – a key document which provides a summary of calculations of benefits and costs.

National Highways initially rejected the request, telling the ICO that the "business case is currently incomplete and not approved by the Chief Secretary of the Treasury", and so sharing it "may cause confusion", it said.

But last month the Information Commissioner instructed it to release its OBC.

It ruled that: "The public interest favours disclosure because the project will have a major and lasting impact on people living and working in that area.

"Those people are entitled to take part in the associated decision-making and to be as fully informed as possible."

“There has never been a more urgent need to tackle the problems at Dartford Crossing..."

National Highways has since abided by that request but the OBC case provided was prepared in August 2020 and is reflective of the proposals at that time.

Since that time a number of changes have occurred including remodelling the opening year from 2026 to 2030.

Revised guidance has also been issued on future growth and has been incorporated into the transport modelling. This has changed the forecast benefits and costs.

Following public consultation last year a number of changes were also made to the proposals.

This included a redesign of the proposals north of the River Thames, to better support the planned Thames Freeport, and modifications to the configuration of the A13 junction, to reduce traffic flow impacts on the local roads in Thurrock.

Procurement has continued and new images have been produced showing how the design of the project has changed since consultation began.

The re-submission was welcomed by the leader of Dartford council Jeremy Kite who says the plans are needed without further delay.

Cllr Kite (Con) said: “The original consultation on the Lower Thames Crossing produced the largest response from the public in Department for Transport history and resulted in overwhelming support for the new crossing.

"I must be honest and say that I think local residents, motorists and anyone who depends on the crossing just want the talking and contemplation to stop and the crossing to be built.

"The delays and hold-ups on the current crossing, not least a recent episode where two protestors on the QE2 bridge were able to disrupt lives and business for two days, is evidence enough that a new crossing is needed and fast.

He added: "The business case is part of the submission made by National Highways to the Planning Inspectorate and it is for the inspector to be satisfied by its contents.”

Mark Bottomley, deputy director of the Lower Thames Crossing, said: “There has never been a more urgent need to tackle the problems at Dartford Crossing, that not only causes frustration for millions of people every year but acts as a handbrake on jobs, growth and productivity.

He added: ”We are designing the new crossing to be the greenest road ever built in the UK, and following a comprehensive programme of consultation and engagement we have improved the design to reduce the impact on Ancient Woodland, avoid protected wetlands, and include new parks, woodland, and pathways for local communities.

"We have now submitted our application for a Development Consent Order (DCO), and our plans will go through a detailed examination by independent, government-appointed experts.

"This is the most robust and demanding planning process and it will give stakeholders and local communities the chance to challenge and scrutinise all aspects of our proposals.”

The Planning Inspectorate says the applicant has agreed "that all application documents can be published as soon as practicable" and it will make these available online "as soon as practicable".

National Highways is expected to find out by the end of the month whether the bid has been accepted, starting the official examination process.

Should it be given the green light it could start as early as 2024 and be finished by 2029.