Tube maintenance staff are to be balloted on industrial action in a dispute over safety.
The Mail, Maritime and Transport (RMT) Union said it was upset Transport for London (TfL) was planning to double, to 28 days, the time between safety inspections of Tube trains.
RMT general secretary Bob Crow said LU was "literally dicing with death in the interests of saving money".
TfL said it would never do anything to compromise safety.
The union said the ballot would be for action short of a strike.
Mr Crow said: "These fortnightly inspections identify worn brake blocks, cracks in securing brackets and other distressed components.
"RMT has a stark choice - ballot for action or sit back and wait for a disaster to unfold. We have chosen to ballot for action."
A TfL spokesman said: "There is absolutely no impact on safety - merely the updating of a maintenance regime that hasn't been changed for decades.
"Our staff have been working with us on this trial for some time and the RMT have been constantly kept aware of the significant progress made. "