The building of a two-mile road tunnel beneath the Thames has been given the go-ahead by the Government.
It was chosen as the preferred option for a new route to ease congestion at the Dartford Crossing.
The link will be the first new crossing of the Thames east of London since the Queen Elizabeth II bridge opened in 1991.
It will 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 project is expected to be completed by around 2025-2027 and has been estimated to cost between £4.3 billion and £5.9 billion.
The Dartford Crossing is used for 50 million journeys annually and is often the cause of frustration for motorists stuck in traffic jams.
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 than there are across the Seine in Paris.
"The new crossing should help to spread the load, as well as bringing welcome regeneration to areas east of the Dartford Crossing.
"For traffic coming from the north-eastern section of the M25, this new routewill give much better and more reliable access to the important port of Dover."