Train drivers on London Underground look set to take three days of strike action over a long-running dispute about pay and plans for an all night Tube service.
Leaders of the drivers' union Aslef will meet on Monday to decide whether to press ahead with stoppages which would cripple Tube services in the capital.
If the executive agrees, there will be a 24-hour strike from 21.14 on Wednesday January 27 followed by two further 24 hour strikes from the same time on February 15 and February 17.
LU has offered a four-year pay deal and said it will hire part-time drivers to staff an all-night service at weekends.
But unions have been seeking assurances for months about terms and conditions for their members.
Finn Brennan of Aslef said: "We genuinely regret the inconvenience that will be caused by any action but the behaviour of London Underground's senior management team have left us with no other choice.
"Our negotiating team last met London Underground at Acas on November 10 and since then they have refused to talk to us despite repeated requests.
"Our members have been extremely patient, they have waited for more than three and half years for promised talks on improving work life balance. There is still no indication when they will receive the pay rise that was due last April.
"We want to see an all-night service introduced, and we are not opposed to the recruitment of part time staff. But we will not accept a zero hours culture being introduced and working conditions undermined. Aslef wants a fair deal for existing staff and for new employees.
"We are ready to talk at any time to finally resolve this dispute. It is time for the mayor of London and his team at TfL to stop the political posturing and engage with us to stop London suffering more disruption."
A spokesman for the Rail, Maritime and Transport union said: "RMT's executive will be taking a report from our London regional organiser on the total breakdown of negotiations over the linked issues of night Tube and pay at a meeting on Monday.
"That report will include a number of recommendations for action in support of our members in this dispute with London Underground.
"Any decisions on action are a matter for the union's executive as set out under RMT's rule book."
The RMT is also in dispute over station staffing issues and is balloting 1,300 Tube Lines maintenance staff over pay and the night Tube.
Steve Griffiths, chief operating officer for London Underground, said: "These latest threats to London show that the Aslef leadership is trying everything to stop the night Tube - a service which will mean easier night-time journeys for our customers, a boost to the economy of £360m, and the creation of around 700 new jobs at London Underground.
"Our four-year pay offer is extremely fair. As a result of hiring new drivers, who will be on permanent, part-time contracts with the same rates of pay and the same benefits as existing drivers, we've made absolute guarantees that no current driver will have to work the night Tube unless they want to.
"This total protection of work-life balance is precisely what Aslef leaders asked for, so it is astonishing that they are now once again threatening strike action.
"The truth is that they are making excessive demands for more money, fewer hours and a four-day week and expect fare and tax payers to pick up the bill.
"That is the real reason they plan to disrupt Londoners. No employer could possibly meet such demands and strike action will change nothing. There is no more money."
London Mayor Boris Johnson said: "It is unbelievable that the leadership of Aslef should now be contemplating strike action.
"They were offered excellent terms and conditions for their members to deliver the night tube.
"They turned those down - without ever properly consulting their membership.
"They claimed - falsely - that the night Tube would badly affect the "work-life balance" of staff.
"In recent weeks LU has advertised for 200 new part-time drivers. Managers have been overwhelmed by the response.
"There have been roughly 6,400 applications. In all logic, Aslef's leadership should simply let this go ahead.
"The new arrangements will, after all, do nothing to affect the existing "work-life balance" of their members.
"But it is clear that they not only want to prevent their own members from doing these jobs. They want to stop anyone else from doing them.
"This is a sickening and cynical display of dog-in-the-mangerism.
"If the strike goes ahead, it will hit many London Underground workers who do not necessarily support the action, and it will cost the strikers in lost pay.
"It will inconvenience the travelling public and it will do absolutely nothing to stop the progress towards the night tube.
"When the new part-time work-force is ready, they will deliver the service. London will get the 24-hour night service it needs, with or without the help of Aslef."
Manuel Cortes, leader of the Transport Salaried Staffs Association reacted angrily to what he said was "politically motivated smearing of the unions" by Mr Johnson.
He said: "Boris himself announced just last month that the night Tube was no longer a priority. Either there's something up with his memory or he's playing a cynical politically motivated smearing game with unions.
"It's not him that delivers the night Tube service it's rail workers and Tube staff and they've never had a problem with it, they just wanted paying fairly for it.
"However Boris's vindictiveness knows no end as he has deprived LU workers a wage rise for the whole of last year as LU management refused to divorce night tube talks from a pay deal.
"Fares have gone up twice now since these talks started but not a single union member has seen an increase in their wages.
"It now looks patently clear that the game of hokey cokey London Underground management have played over the last year with night Tube on and night Tube off was devised by Boris in City Hall."
Mr Johnson added: "It would have been much better and quicker if the union leadership had even allowed their membership to consider the excellent offer from LU, but they seem to have succumbed to an advanced attack of Corbynitis."
Liberal Democrat mayoral candidate Caroline Pidgeon said: "The Mayor and TfL have messed up this negotiation by giving an artificial deadline for the start of the night Tube, allowing the unions to hold them to ransom.
"But given that date passed last September, it is quite clear that the unions are now playing games rather than working all out to secure an agreement.
"Both sides need to come together, sit round the table and hammer out a full and fair deal. Londoners need, and expect, nothing less."
Transport secretary Patrick McLoughlin said: "Threatening disruption for Londoners so Union fat cats can enjoy a four day week is selfish and irresponsible. A 24 hour Tube will create jobs and allow a major economic boost for London - it's an idea whose time has come. Aslef want to hold back progress for a set of totally unreasonable demands."