Nick Raynsford, the minister for London, will end months of speculation when he announces today that the London Bridge City project has been chosen to house the mayor's office, an assembly chamber and the authority's 400 staff.
Sir Norman Foster's 10-storey development on the Thames will become the headquarters of the Greater London Authority when it is completed next year, The Independent can reveal.
Whether Ken Livingstone, Jeffrey Archer or Mr Raynsford himself will inaugurate the building as Britain's first directly elected mayor remains to be seen, but the Government is satisfied the project is a fitting home for the authority.
The landmark new building was seen by Tony Blair as the ideal site because of its stunning views over the heart of the capital and its proximity to Tower Bridge, a symbol of the city across the globe. The development, which will be ready six months after the authority elections in May 2000, features a new pier to allow the mayor and visitors to arrive by boat.
Ministers were impressed by Sir Norman's futuristic design, with fully glazed facade to allow the assembly chamber a clear view of the river, and believe it will become a 21st-century icon reflecting the authority's "go-ahead" spirit.
The building will have an open-top roof terrace reached by two enclosed glass lifts that travel outside the building. A 10th-floor gallery, described by Foster and Partners as "London's Living Room", will host major events and banquets of up to 200 people.
Close to the new Jubilee Line Tube station at London Bridge, the building was also selected because of its excellent transport links and environmentally friendly design.
The scheme has beaten the other contender for the authority site, a classical office block near the British Museum in Bloosmbury. The two projects were whittled down from an original list of 50 last year.
The Bloomsbury scheme was withdrawn temporarily from the bidding earlier this year when English Heritage objected to its design, but was resubmitted with support from its local council, Camden.
Niall Duffy, leader of Southwark council, said he was delighted by the move as it would provide up to 10,000 jobs for local people.
"From the outset, we have been convinced that Southwark and this very special site are the ideal location for a landmark building that represents London's aspirations for the 21st century," he said.
The Greater London Authority will have an annual budget of pounds 3bn and take responsibility for public transport, planning and policing in the capital from 3 July 2000.