The Tube network is one of London's most iconic symbols
London Underground has revealed it is to cut 1,000 jobs.
The firm said no front-line staff or Tube drivers would go but the cuts would come by cutting temporary and administrative posts.
Union leaders vowed to fight the cuts and described the announcement as a "body blow". They said the lost jobs would lead to service cuts.
More jobs are also expected to be cut at the wider Transport for London (TfL) network that includes the bus system.
A TfL spokesman said: "Discussions with unions about London Underground's operating cost review started today.
"Front line operations, including train, station and maintenance staff are not part of the review and will be unaffected by any change."
The spokesman said vacant positions will go unfilled and temporary posts will not be renewed in a bid to cut the number of staff while avoiding mandatory redundancies.
Jobs set to be affected include those in finance, IT, procurement, human resources, legal and administration.
The Underground employs about 20,000 staff.
Gerry Doherty, leader of the Transport Salaried Staffs Association, said: "This is a real body blow for staff and the London travelling public.
"There is no way that cuts of this size will not hit services. We should be investing in the Underground in the run up to the Olympics, not cutting back.
Bob Crow, general secretary of the Rail Maritime and Transport union (RMT), said: "Any attempt to impose compulsory redundancies among our members or to undermine their terms and conditions will be met with a ballot for industrial action."
London's Tube network covers more than 400km (250 miles) of track and each year it is used for more than one billion journeys.