Tube maintenance workers will stage two 48-hour strikes in a protest over pay and conditions, it has been announced.
RMT union members will walk out at 1900 BST on 23 June and 14 July in a strike they say will have "severe consequences" across the Tube system.
The Piccadilly, Northern and Jubilee Lines are expected to be hardest hit.
Transport for London (TfL) called for the strikes to be postponed and "proper dialogue" to be held. It is to take over maintenance by the end of July.
At the moment the work is done by private contractor Tube Lines, which has been criticised for long delays in upgrades.
RMT 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 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."
Northern Line work
But in a statement, TfL said this was "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 and to enter proper dialogue with us after our acquisition of Tube Lines."
On Tuesday BBC London revealed many track faults had been left for up to three years after being reported without being fixed by Tube Lines.
Tube Lines has insisted its record was good.
Meanwhile upgrade work by Tube Lines on the Northern Line will see many stations shutting at 2130 BST on Monday to Thursday evenings throughout July and August.
This will affect the Bank branch between Camden Town and Kennington and the High Barnet branch north of East Finchley.
The Charing Cross branch will continue to run until the usual weekday evening time, however.
There will also be five weekend closures, it has been announced.