The Rail Maritime and Transport (RMT) union secretary Bob Crow and Transport Salaried Staffs Association (TSSA) leader Gerry Doherty are due to meet today to finalise the timetable for industrial action.
The first strike is expected to begin at midnight on 7 September coinciding with the first week of the new session of Parliament.
The London Chamber of Commerce said the strikes could cost the city £48m a day.
London Underground said there would be no compulsory redundancies from its plans to reduce ticket office opening hours, and chief operating officer Howard Collins urged the unions to call off the strikes.
Both parties have spoken to conciliation services group Acas.