RMT has confirmed a further two 24-hour strikes over the Night Tube to take place in August.
Train operators and instructor operators will walk out from 9pm on Tuesday 25 August until the same time 26 August. It will be repeated again from 9pm on 27 August until 28 August. All other staff will down tools from 6:30pm on both days.
Unions including TSSA and Unite are also taking part in the action.
The decision comes after talks at conciliatory service Acas were adjourned yesterday afternoon.
The dispute centres around the introduction of the Night Tube: unions RMT, TSSA, Aslef and Unite claim that Transport for London has failed to offer sufficient guarantees around staff's "work-life balance".
RMT general secretary Mick Cash said: “Staff are angry that their grievances are being ignored and dismissed by both the mayor and his officials and that there has been no substantial move as of today from London Underground to make progress towards a negotiated settlement. That is why we have no option but to strike again.
“The Night Tube plan has been botched from the off. The basics haven’t been done and those who will pay for this shambles will not only be our members but the London daily travelling public who cough up a fortune and who will find their safety and the reliability of the service compromised from 12th September onwards. Running Tube services with fatigued and burnt-out staff is a recipe for disaster.
TSSA general secretary Manuel Cortes blamed mayor Boris Johnson for the stand-off.
He said: "It is now time for the mayor to start getting his hands dirty and get down to serious negotiations to avoid a week of disruption on the Tube. He should stop playing Pontius Pilate. He will bear a heavy responsibility if the 48-hour stoppage goes ahead while he washes his hands of his crucial role as the democratic leader of London."
"He will blame the rail unions for the disruption but the travelling public will be asking why he has allowed these talks to drag on for six months whilst he refused to meet the union leaders themselves.
"Our members will not accept top down change being imposed on them when it comes to changes in their work-life balance. We accept the Night Tube but only with detailed agreement on how it will operate."
Read more: This is the pay deal the unions rejected
But Steve Griffiths, London Underground's chief operating officer, was robust in his response.
"Having previously argued that it was ‘not about money’, the RMT leadership [has] made further demands for unaffordable pay rises and the hiring of even more staff – including the reversal of the modernisation of customer service in LU stations," he said. "They expect our customers to pick up a bill running into hundreds of millions of pounds for this through higher fares or by forcing us to cut back investment to improve Tube services in a rapidly growing city. No responsible management can give in to such demands."
Initially the Night Tube will run for just two days a week - Friday and Saturday nights - and on just five of the 11 Tube lines (Jubilee, Piccadilly, Central, Northern and Victoria), meaning there will be no need for staff to work longer hours and annual leave entitlements will remain the same - 43 days a year for drivers and 52 days a year for station staff - as will the fact staff have two days off in seven.
Griffiths added: "Our pay offer is fair and realistic, including an above-inflation increase this year, inflation protected increases in the next two years and a range of bonus payments for the Night Tube.. Instead of yet more strike threats, we need level headed and sensible discussion on how to deliver a modern transport service for London, alongside fairly rewarding our hardworking staff and protecting their work-life balance.
"That deal needs to be affordable and sustainable – and certainly not be at the expense of massive fares rises or cuts to Tube improvements – and can only be agreed around a negotiating table, not through strike threats. We remain ready at all times to meet and negotiate such an agreement for the benefit of our staff, customers and London as a whole.
"We urge the unions to join us."