A last-gasp bid to prevent a London Tube strike on 8 and 9 July failed after London Underground (LU) and the unions failed reach an agreement over pay and conditions.
Drivers and other workers will walk out from 6.30pm BST and there will be no service on the Tube for 24 hours.
LU bosses and delegates from four unions representing 20,000 met on 7 July to try to thrash out a deal over the introduction of a new night service on several lines from September. However, the the unions said no deal had been reached.
The Rail, Maritime and Transport union (RMT), Transport Salaried Staffs Association (TSSA), and Unite members plan to walk out at 6.30pm BST. Aslef drivers will begin strike action from 9.30 BST. No further negotiations are planned, according to RMT.
Mick Cash, RMT general secretary, said: "Despite strenuous efforts by union negotiators to press London Underground to address the issues of fairness, safety, work-life balance and equality at the heart of this dispute, they have come up with nothing in the talks this afternoon."
He added the "action is on" and "there are no further talks planned at this stage".
Transport for London (TfL) said extra bus and river services would be run on 9 July, but warned that roads and all public transport would be much busier than usual.
Full and final offer
LU chief operating officer Steve Griffiths said in a message to staff that a "full and final" offer had been made in an attempt to conclude five months of talks. The BBC said the offer was said to include an average 2% increase on basic pay, RPI inflation or 1% (whichever is greater) in 2016 and 2017, and a £500 non-consolidated "launch bonus" to all staff on night Tube lines or at stations serving those lines.
He said: "Many of our staff will not be affected by the new services as we are operating Friday and Saturday night services on five lines. We are also hiring 137 more train operators specifically to work on the night Tube.
"For those who are affected, it will mean a few extra nights per year within the existing working week. No one is being asked to work more hours than they do now. We are not asking staff to work unlimited nights and weekends."