London's Mayor is being urged to intervene in the deadlock over an all-night Tube service in the capital.
Boris Johnson is being invited to meet union leaders following the failure to reach a deal at long-running talks with London Underground.
The night Tube was due to start last month but unions have said it will not now be launched this year.
Some officials are starting to question if it will begin before the London mayoral election next May.
Manuel Cortes, general secretary of the Transport Salaried Staffs Association, said in a letter to the Mayor that unions were "frustrated " that no agreement has been reached.
"The night Tube should happen and, given that we all want it to happen, it should be possible.
"This requires you to put your Conservative Party leadership ambitions on hold for a short time and give all your efforts to the most immediate job in hand," wrote Mr Cortes, calling for face-to-face talks.
LU has threatened to go over the heads of union leaders to consult workers on a pay and conditions offer for the planned new service.
The Mayor's official spokesman said: "The Mayor and London Underground have been clear that the introduction of Night Tube will not be at any cost.
"The unions have failed to put what is a very fair and sensible offer to their members.
"The Mayor has made clear negotiations are a matter for London Underground management. To do anything else risks undermining that process.
"Given the unions know that, it's clear to the Mayor that they are intent on needless headline grabbing rather than doing what they should be doing, and putting the deal to their members."