Tube workers plan to stage a 48-hour strike in a row over staff transfers and pensions.
The strike will begin at 1030 BST on Monday 28 April, the Rail Maritime and Transport (RMT) union said.
It said Transport for London (TfL) had not provided written guarantees on pensions and travel provision for 2,500 staff at failed private firm Metronet.
TfL said it had assured the RMT that no staff would lose jobs or pensions, nor would any be transferred.
The RMT wants a guarantee that Metronet workers will be allowed to join the TfL pension scheme and receive the same travel facilities as other TfL employees.
RMT general secretary Bob Crow said it had only received "hedged, qualified and ambiguous" guarantees from TfL.
"The RMT executive was left with no choice but to set strike dates," Mr Crow said.
'Shameful behaviour'
Tube maintenance firm Metronet went into administration last July after an estimated £2bn overspend.
Last month, the House of Commons Transport Committee said the company and its shareholders were "primarily responsible" for its collapse.
Mr Crow said the union was "adamant" that its members would "not be made to pay" for the collapse of Metronet and the "shameful behaviour" of its shareholders.
"The shareholders who walked away from Metronet's corpse are being rewarded with fat PFI (Private Finance Initiative) contracts, yet the people who have stuck with the job of improving the Tube are supposed to accept uncertainty over their jobs, pensions and conditions. That is not on," Mr Crow said.
"We have already made it clear that the collapse of Metronet should not be used as a Trojan horse for a two-tier workforce."
'Completely unnecessary'
Last September a strike by Metronet workers closed two-thirds of the Tube network and affected 10 lines.
A TfL spokesperson said on Friday: "A strike at Metronet is completely unnecessary and will not achieve anything."
He said TfL had already guaranteed the RMT that no staff would lose jobs, pensions or be transferred as a result of Metronet's collapse.
"Indeed, the vast majority of Metronet employees already enjoy these benefits," the TfL spokesman added.
He said TfL had reached an agreement with the RMT "on all these issues", which stands while Metronet continues to be in administration.
"We have no doubt that these issues can be resolved," the TfL spokesman said.
An RMT spokesman said no further talks on the issue have been planned between the union and TfL.
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