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A union source has told IBTimes UK that optimism has died down over the Night Tube talks Getty

The long-awaited Night Tube will not launch until 2016 unless London Underground (LU) bosses make a "dramatic change" in negotiations, a union source told IBTimes UK. The warning comes as Aslef, TSSA, the RMT and Unite continue to talk with LU management in a dispute over pay and work conditions.

The unions are unhappy about work rosters and pay proposals set out by Transport for London (TfL) for the new service, despite all workers being offered a 2% salary increase for 2015 and drivers being offered an extra £200 ($306) per Night Tube shift.

Aslef, which represents a majority of drivers on the network, and the TSSA, which represents Underground workers, had previously reported progress in the negotiations over the work/life balance issue. But the union source told IBTimes UK that optimism had died down since September, putting a 2015 roll-out of Night Tube under threat.

The delay would be another setback for TfL after it pushed back the Night Tube's original launch date of 12 September amid the threat of strikes from the unions. However, a TfL spokesman told IBTimes UK that Underground bosses are still working towards an autumn launch and hope to find a resolution with the unions. The comments come after IBTimes UK exclusively revealed on 1 September that it would be "unlikely" for the new service to launch before Christmas.

The Night Tube has been hailed as being "pivotal to London's night economy" by TfL and the company said it will boost the capital's economy by £360m. The service will run on major lines, including the Northern, Piccadilly and Central lines, and is expected to support 2,000 permanent jobs.

TfL said: "The new service has been made possible thanks to the continued modernisation of significant parts of the Underground network and improved levels of reliability. Independent research into the economic benefits of the Night Tube found that the new service will boost jobs and help maintain London's status as a vibrant and exciting place to live, work and visit."