Transport for London is looking to slash up to 1,500 jobs because of spending cuts, according to unions.
Jobs in engineering and parts of London Underground could be in jeopardy, the Rail, Maritime and Transport union warned.
It comes after a meeting between TfL and the unions on Tuesday.
TfL bosses indicated that they were looking to make £5.5 billion worth of savings by 2021, RMT claimed.
TfL has disputed the number of job losses this would incur.
The union criticised the move, which comes days before the 30th anniversary of the deadly King's Cross fire which killed 31 people.
RMT general secretary Mick Cash said: "All the Tube and TfL unions have been sent written details of the next phase in the wrongly named transformation process.
"As part of the Mayor's efforts to slash spending by £5.5 billion by 2021 we are now being told that the company plan to cut up to 1,400 jobs in engineering in TfL and in some areas of LU.
"RMT demands that no cuts take place, so close to the King's Cross fire anniversary. It would be appalling if there was any hacking back on safety.
"The Mayor needs to stand up for TfL and demand restoration of the full capital grant and proper central government funding for the Tube."
A TfL spokesman said the body disputed how many job losses would result from the shake-up, but insisted it would not affected passenger safety.
"We are undertaking the largest ever overhaul of our organisation to provide the most efficient and cost effective transport service for Londoner," the spokesman said.
"We have already reduced management layers and bureaucracy and merged functions in other areas to eliminate duplication and reliance on expensive agency staff."