Fresh talks are being held in an effort to avert a series of strikes by London Underground.
Leaders of Aslef, the Rail, Maritime and Transport union and Unite will sit down with LU bosses on Friday at the conciliation service Acas.
Three 24-hour strikes have been called in the coming weeks in the long-running dispute over pay and plans for an all-night Tube service.
Finn Brennan, Aslef's organiser on London Underground, said: "A world class capital city like London deserves a 24-hour service but it needs to work for those who provide that public service, as well as for the passengers who use it, and the management at TfL. London Underground needs to stop bullying and start negotiating."
An RMT spokesman said the union will attend the fresh talks in the hope of breaking the deadlock.
TSSA leader Manuel Cortes urged London mayor Boris Johnson to meet unions, adding: "All Tube unions have ever wanted is to reach an agreement with London Underground management that brings LU's once-flagship night-Tube policy on line for them without compromising passenger safety standards nor the work-life balance of our members."
The unions are planning three 24-hour strikes from the evenings of January 26, February 15 and 17.
Additional reporting by Press Association.
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