The consortium renewing parts of London Underground is to axe hundreds of jobs under a "streamlining" plan.
Metronet, responsible for maintaining and overhauling two-thirds of the Tube system, said 290 administrative and middle management jobs will be affected, as well as up to 200 temporary and agency posts.
The Rail Maritime and Transport union immediately warned it will be in dispute if any of its members' jobs are cut.
Metronet, which is seeking a review of its public private partnership (PPP) contract because of escalating overspending, said it will redeploy at least half the 290 admin and management posts.
Chief executive Andrew Lezala said: "The ongoing need for Metronet to be economic and efficient continues to be a key business objective, and it addresses issues raised by the PPP arbiter in his annual review last November.
"Whilst I have already introduced significant change within Metronet, it's been necessary for us to conduct a further review to ensure that we have the right number of people in place, doing the right jobs in our business."
Metronet said it will streamline its administrative support functions, reduce the number of management levels between senior managers and the shop floor and restructure its track and signals maintenance operation.
Metronet, whose shareholders are Balfour Beatty, Atkins, Bombardier, EDF Energy and Thames Water, said it will also withdraw unfilled vacancies and lose jobs through natural attrition.
The consortium said the financial over-run on the contract was higher than the £750 million revealed last year, with some estimates putting the financial black hole as high as £1.2 billion.
Following the changes, Metronet said it will employ 5,250. Separately, the consortium said it had completed a recruitment programme of 550 frontline engineering staff in its maintenance operation.
