Thousands of London Underground workers are to vote on whether to continue a campaign of strike action in a long-running dispute over jobs and pensions.
More than 10,000 members of the Rail, Maritime and Transport union (RMT) will be re-balloted in the coming weeks.
Unions have to achieve a fresh mandate for industrial action every six months.
The union says other plans have raised the prospect of Tube workers losing over 30% of their pensions.
The ballot will run until the May 23 and the current mandate runs out in June.
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.
“Transport for London 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.”