London Underground (LU) has said it will cut an extra 400 office and management jobs and leave an equivalent number of vacancies unfilled.
It comes after the second of a series of strikes in protest at 800 ticket office jobs Transport for London previously said it will cut.
LU says it is reviewing all non-frontline services to try and save money.
The Rail Maritime and Transport union called the latest cuts "savagery".
General secretary Bob Crow said: "This is yet another tranche of job losses even before LU's wider review is finished and comes ahead of the government's spending review announcement later this month.
"This is very much the thin end of the wedge and underlines how right we are to challenge the job cuts."
LU said in a statement: "LU is reviewing all non-frontline services to ensure the right levels of resources are in place.
Frontline 'not affected'
"The review will reflect recent changes which have led to reductions in activity in some back-office areas.
"Customer facing and frontline roles, such as train operators, station and maintenance staff are not affected by this review. Around 400 permanent employees in back-office roles will be affected."
Two strikes are planned for November over the 800 ticket office jobs.
The mediating service Acas has sent out a formal invitation to LU, the RMT and the Transport Salaried Staffs Association to attend fresh talks on Tuesday to resolve the dispute.