The idea of a tunnel crossing was first proposed by the Ministry of Transport in 1924. Initial reports at the start of the year suggested a crossing between Tilbury and Gravesend, replacing a ferry service, but this had been rejected by July in favour of a route further upstream, near Dartford. By 1929, the total cost of building the tunnel had been estimated at £3 million (equivalent to £ million in ). The tunnel was planned to be part of a general orbital route around London and was provisionally known as part of the "South Orbital Road".