Over 10,000 RMT members across London Underground will be re-balloted for strike action from today in a dispute over jobs and pensions.
The tube union has been locked in a row with Transport for London and the Mayor for nearly a year, following plans to cut 600 jobs on stations with many more across train operations and maintenance to follow.
Pension plans have raised the prospect of tube workers losing over 30% of their pensions.
Mayor Sadiq Khan is under pressure from the Tory government to cut £1bn from TfL operating budget.
RMT has always insisted that the Mayor should stand firm with tube unions to resist the ideological cuts being made by Conservative ministers.
TfL has forecast a £75m surplus on budget this year and passenger numbers are rising faster than forecast.
The ballot will run from 25 April until the 23 May and the current mandate runs out in June.
Speaking as the re-ballot opens, RMT general secretary Mick Lynch said: “Our members have taken several days of strike action over this last year and remain as determined as ever to get a just settlement on jobs, pensions and their working conditions.
“The Mayor is under pressure from central government, but he must join us in resisting them and refuse to allow ideologically motivated financial constraints to be used as an excuse to attack tube workers.
“TfL has healthy revenue streams and our members are among the thousands of tube staff that make it a successful transport provider.
“This re-ballot is vital to maintain the pressure on TfL and I urge all our members to vote yes in the postal ballot.”