Contractor Osborne will start initial non-disruptive preparatory work next year under a £10m-plus contract.
This will involve removing and transferring key station services and assets, such as telecoms cabling, around the station by 2013.
Costain will then start preparatory work on the station after the Olympic Games next summer, with major work on its £400m contract due begin in 2013.
The station will remain operational throughout the redevelopment and will be complete in 2018.
David Higgins, Network Rail’s chief executive, said: “London Bridge is one of the busiest stations in the country and this investment is vital for passengers who want more space, less congestion and better services.
“Our proposals will breathe life back into London’s oldest rail terminus and help build on the programme of regeneration along the South Bank and Bankside. It will provide a focal point for the local communities it serves as well as the tens of millions of passengers who use it every year.
“We are grateful for the support we’ve received and will continue to work closely with our partners and the local authority to deliver the project efficiently and considerately over the coming years.”
A key part of the £6bn Thameslink programme, plans to redevelop London Bridge include a completely new platform layout, which will allow a metro-frequency of trains on the Thameslink route and the creation of Britain’s largest station concourse, providing more space and reduced congestion for passengers.