Tube workers will stage two 24-hour walkouts in the ongoing saga over all-night trains.
The Rail, Maritime and Transport (RMT) union said its members will strike for 24 hours from 6.30pm on August 25 and again from 6.30pm on the 27th.
The industrial action is set to cause chaos on public transport.
It comes as fed up commuters have endured two strikes in the past month over plans to start 24-hour trains from September 12.
Unions members are angry about pay and shift arrangements for the new weekend service.
They have voiced concerns about how "rosters from hell" will affect their "work/life balance."
RMT general secretary Mick Cash said: "Our members the length and breadth of London Underground have made it clear that they are determined to protect their work/life balance and not fall into a trap where they will be forced to wreck their home lives in order to comply with the 'rosters from hell' which have been drawn up to plug the staffing gaps in the night Tube plans.
"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. It is ridiculous that the job cuts programme for fleet, engineering and station staff is also still being bulldozed through against this background.
"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 September 12 onwards.
"Running Tube services with fatigued and burnt-out staff is a recipe for disaster."
But Twitter users had little sympathy for the Tube workers, with many accusing the union of "holding the capital to ransom".
Steve Griffiths, London Underground's (LU) chief operating officer, insisted an "extremely fair offer" had been made and that they were "prepared to be flexible to reshape it to best suit our staff."
The strike last Thursday (August 6) led to around 250 extra buses being laid on as millions of commuters battled their way into the city.
