London Underground maintenance workers are to stage two 48-hour strikes in a row over jobs, pay and conditions.
The Rail Maritime and Transport union said its members working on Tube Lines contracts will walk out from 7pm on June 23 and again from 7pm on July 14.
The union said it had been unable to receive assurances on jobs, working conditions and pay as the company's work transfers to Transport for London.
The union warned that the strike will have "severe consequences" across the Tube network, especially the Piccadilly, Northern and Jubilee lines, which are maintained by Tube Lines.
General secretary Bob Crow said: "We have given TfL, and their newly-acquired subsidiary Tube Lines, plenty of opportunity to give us assurances that staff won't take the hit for the failure of the doomed privatisation project.
"They have failed to give us those assurances, leaving us with absolutely no option but to announce these strike dates to protect both our members and the safety of the travelling public."
TfL announced last month it was taking over the Tube Lines work, signalling an end to the public private partnership (PPP) arrangement on the Tube.
RMT members have voted by more than 9-1 for industrial action.
A TfL spokesman said: "As the RMT leadership knows, we do not currently control Tube Lines. We are working to conclude the acquisition of the company by the end of June, which will benefit all Londoners by effectively ending the PPP, which has wasted hundreds of millions of pounds and led to delays to upgrade work.
"The RMT have time and again called for Tube Lines' work to come back under public control and that is what we are working to achieve. Furthermore, this is an existing dispute about pay and conditions and not about the proposed transfer of Tube Lines to TfL. We strongly urge RMT leadership to call off the threatened strike action."
