Talks aimed at averting a 72-hour strike by London Underground maintenance workers next week ended without agreement tonight.
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 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.
The announcement follows a 4-1 vote in favour of industrial action in a ballot of union members.