The Bank Station capacity upgrade project will create a new ticket hall at surface level.

The successful bidder will also carry out tunnelling work to divert the southbound Northern line running tunnel and build a new southbound platform to the south-west of the existing platforms.

Bank is one of busiest and most complex interchange stations on the network, in the vicinity of busy road junctions and through routes for traffic, next to several Grade 1 listed properties.

The ambitious job to raise capacity at the crowded station has between given a price tag of anywhere between £250m and £500m.

London Underground is calling for expressions of interest early in the process to allow the selected firm or consortium to plug its engineering expertise into initial designs.

The winning bidder will be brought on board in the Summer of 2013 with a timetable to start works in 2015 for completion at the end of 2021.

Richard Parry, London Underground’s strategy & commercial director, said: “Currently the Northern line platforms at Bank in particular suffer severe congestion during morning and evening peak times, which will get worse as passenger numbers grow.

“This major project to re-tunnel the Northern line at Bank, making more spacious platform areas, with a new entrance on King William Street and four high capacity lifts will relieve congestion at the station and provide step-free access for the Northern line platforms.”

Firms interested in bidding for the huge underground project have until 19 December to submitted expressions of interest to [email protected]