A tunnel beneath the River Thames is expected to be announced as the Government's preferred option for a new road crossing.
Transport Secretary Chris Grayling will confirm the chosen route on Wednesday.
Highways England said in January 2016 that it supported the tunnel proposal, connecting Gravesend, Kent, with Tilbury, Essex.
The organisation, which is responsible for operating and improving England's motorways and major A roads, rejected a shortlisted option to build a bridge or tunnel next to the existing Dartford Crossing.
Highways England said its favoured option would reduce pressure on the Dartford Crossing and offer a shorter route to and from the Port of Dover and the Channel Tunnel.
It would run from the end of the M2 near Rochester, cross the river to the east of Gravesend and join the M25 between junctions 29 and 30.
The Dartford Crossing is used for 50 million journeys each year.
AA president Edmund King said: "The case for greater road capacity across the Thames is overwhelming and overdue.
"There are half as many crossings of the Thames then there are across the Seine in Paris.
"Currently there is often congestion in both directions on the M25.
"The AA argued for greater capacity but didn't take a stand on a specific route as we believed this is a decision for local people to influence in the consultation."
Three potential approach routes have been identified north of the river in Essex and two south of the river in Kent. The tunnel has been estimated to cost between £4.3 billion and £5.9 billion.
An estimated 77,000 vehicles would use the link each day in its first year.
It would be the first new crossing of the Thames east of London since the Queen Elizabeth II bridge opened in 1991.
Steve Gooding, director of the RAC Foundation, said: "Drivers will hope ministers won't skimp on the design for the new crossing.
"With traffic on the motorway network predicted to soar in the years ahead, motorists will want reassurance that the scheme is as future-proof as possible and will add as much capacity as is practical.
"This will be doubly true if we have to pay for the privilege of using the new crossing as we currently do at the old one."
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.