A third union will join Aslef and RMT in taking part in Tube strikes, the first of which is due to take place next week if an agreement can't be reached over pay and working conditions connected to the Night Tube.
TSSA has said it will also walkout if talks between the unions and Transport for London are not resolved, just a day after Unite the union pulled out of the planned action.
"TSSA members want to go forward and deliver a safe 21st-century Tube system benefiting our great world city but they fear Boris Johnson's management of the Tube is taking it back to 19th-century deregulated safety standards and working practices," said TSSA general secretary Manuel Cortes.
Talks will resume again today with conciliation service Acas to avert the strikes which would bring London to a halt. "Productive" meetings began on Friday, bringing the two sides together for the first time since November.
The first strike is scheduled for Tuesday 26 January, starting from around 6pm. Second dates are scheduled for Monday 15 February and Wednesday 17 February.