Talks aimed at averting a 72-hour strike by London Underground (LU) maintenance workers next week have ended without agreement.
Leaders of the Rail Maritime and Transport (RMT) union and management from LU met at the conciliation service Acas to try to resolve a row over pensions.
A joint statement said: "Both sides are disappointed that, despite seeking opportunities for resolution, no agreement has been possible at this stage."
No further talks have been arranged, raising the threat of disruption to Tube services next week.
The union, which represents around 1,000 staff on the Tube Lines contracts, announced earlier this week that its members will walk out at 4pm on Tuesday until the same time on Friday. The announcement followed a 4-1 vote in favour of industrial action in a ballot of union members.
The staff work on the Piccadilly, Northern and Jubilee lines and provide a number of services, including emergency repairs, across the entire network.
The row is over union demands for all Tube Lines staff to be allowed to join the Transport for London pension scheme and receive the same travel concessions as those who work for London Underground.
Jon Lamonte, Tube Lines' chief executive said: "Earlier today Tube Lines and the RMT met at Acas to try to resolve this issue through constructive discussion. Unfortunately the RMT leadership have not agreed to suspend their action, which is completely unnecessary and will not achieve anything. Since becoming part of Transport for London, Tube Lines' employees have kept their existing terms and conditions for all staff including pension and travel.
"The RMT's ballot showed that less than 44% of its members voted in favour of strike action. We urge the RMT leadership to return to discussions, and we remain ready to continue such discussions at any time to try and avert the strike. Should this industrial action go ahead, we expect it to have little impact on the Tube services. We have well practised contingency plans in place to ensure that the essential maintenance required on the railway can either go ahead as planned or can be rearranged. We do not and will not put passenger or staff safety at risk."
Bob Crow, leader of the RMT said: "We entered these talks on equality and fairness in good faith but the management offered nothing and made a farce of the whole process. The action next week remains on and it is the management's intransigence that is responsible for the disruption travellers can expect."