A major new road incorporating the Lower Thames Crossing will be a three-lane dual carriageway throughout to almost double road capacity across the river, Highways England has announced, as the revamped £6bn scheme goes out to fresh public consultation.

The government says the controversial link from Essex to Kent, billed as Britain’s most ambitious road scheme since the M25 motorway orbiting London was built, will boost south-east England’s economy and tackle congestion across the Thames east of London when it opens in 2027.

Campaigners and councils have strongly opposed the scheme, which threatens to destroy wetlands and increase car use and is being pushed through immediately after UN warnings of catastrophic climate change.

The planned 14.5 mile (23.3km) road between the M25 and the M2, which serves the Channel ports, will run through a 2.4 mile tunnel under the Thames, the longest road tunnel in the UK and the third widest bored tunnel in the world.

It is expected to divert 14m vehicles a year from the Dartford Crossing, the only road crossing east of London, cutting northbound journey times around the M25.

The preferred route was approved by the transport secretary, Chris Grayling, in April 2017. Since an initial consultation attracted 47,000 responses, the plans have been revised with some mitigation measures for local communities, including extending the tunnel by 600 metres to move the entrance in Kent further south and lowering parts of the new road by more than five metres to reduce its visual impact. The new consultation will run until 20 December.

Grayling said the crossing would “help transform journeys, create new business opportunities in Kent and Essex and unlock productivity across the UK … and cut congestion at the Dartford Crossing and improve connectivity from our industrial heartland to our ports in the south-east.”

An automated Dart Charge scheme to pay the £2.50-£6 vehicle toll has helped cut congestion at the Dartford Crossing, but it remains a notorious traffic blackspot. The Highways England project director, Tim Jones, said it remained a vital gateway but carried more traffic than it was designed for: “Drivers there suffer from regular delays, severing communities and holding back economic growth.”

Hauliers and motoring organisations have strongly backed the new crossing. The Freight Transport Association said congestion at Dartford was “unbearably high” and that the M2-M25 route needed to work as smoothly as possible to ensure that British companies could trade domestically and internationally.

But the Campaign for Better Transport described the scheme as “bad value” and likely to shift Dartford’s congestion and pollution problems downstream, saying that “practical alternatives are available, at a fraction of the financial and environmental cost”.

The Friends of the Earth campaigner Jenny Bates said: “It’s astonishing that in the week the UN is warning of the need for urgent action to avoid catastrophic climate change, the UK government is boasting about building more roads. This development will only encourage more cars, vans, lorries and traffic, pumping more air pollution and climate-damaging emissions into our environment.”