The Lower Thames Crossing is billed as the UK's most ambitious road scheme since the M25 motorway opened 30 years ago, and will include Britain's longest road tunnel.
Transport Secretary Chris Grayling said the toll road will "transform journeys, create new business opportunities in Kent and Essex and unlock productivity across the UK" when it opens in 2027.
A 10-week public consultation on the latest designs for the scheme opened this week.
The planned new 14.5 mile route will connect the M2 near Rochester in Kent and the M25 in Essex between North and South Ockenden.
It will also include a 2.4 mile tunnel between Gravesend and Tilbury - the longest road tunnel in the UK.
But public transport groups claim ministers should instead focus on improving rail links. There are also fears that traffic using the new crossing will blight communities with pollution on approach roads.
The tunnel is expected to divert vehicles from the Dartford Crossing, the only road crossing of the river east of London. It is used for 50 million journeys annually and is often the scene of frustration for motorists stuck in traffic jams for several hours following accidents.
The new crossing is expected to reduce traffic at Dartford by 22 per cent, with 14 million fewer vehicles using it every year.
And the route will almost halve the morning peak average journey times between M25 junctions 1b and 31 from nine minutes to just five, Highways England said.
Under the latest designs, the whole route of the Lower Thames Crossing will be a three-lane dual carriageway rather than two lanes in some sections. This will result in a 90 per cent increase in capacity for traffic crossing the Thames east of London, compared with around 60 per cent - 70 per cent under the previous two-lane plan.
The scheme's budget has risen to a range of £5.3 billion to £6.8 billion due to more detailed design work and ground investigations. A previous estimate put the range at £4.4 billion to £6.2 billion.
The design has been updated to "minimise the impact on local communities and environment", Highways England said. Other changes in the latest designs include:
- A new rest and service area to the west of East Tilbury
- Removal of the proposed Tilbury link road to reduce traffic on the local road network
- Lowering the road by up to six metres in places to reduce its visual impact
Transport Secretary Chris Grayling said: “We are making the biggest investment in improving our roads since the 1970s, ensuring journeys are quicker and safer. The Lower Thames Crossing will help transform journeys, create new business opportunities in Kent and Essex and unlock productivity across the UK.
“It will also cut congestion at the Dartford Crossing and improve connectivity from our industrial heartland to our ports in the South East.”
The scheme's project director, Tim Jones, said: “For too long the Dartford Crossing has been the only way to get across the Thames east of London. It is a vital gateway, but carries more traffic than it was ever designed for and drivers there suffer from regular delays, severing communities and holding back economic growth.
"The Lower Thames Crossing is the most ambitious project of its kind ever in the UK and the biggest single road upgrade since the M25 was completed more than 30 years ago.
"It would almost double road capacity across the Thames cutting congestion, significantly easing pressure at the Dartford Crossing and boosting the resilience of the whole road network. This consultation is an important opportunity for people to share their views on our proposals and I urge anyone who has an interest to have their say."
To have your say on the consultation, visit www.lowerthamescrossing.co.uk/haveyoursay