Tube and other transport workers on London networks will vote in the coming weeks on whether to launch a campaign of industrial action.
Members of the union Unite at Tfl and the Underground will vote on whether to launch a fresh strike in mid-June.
It comes after a string of strikes earlier this year, and ongoing Night Tube strikes by Tube workers disputing rota changes.
Unite said workers will vote on whether to strike after they were told that the value of their pensions will be cut and a final salary scheme will end following a central government-demanded review in return for pandemic-recovery funding.
Unite regional officer Simon McCartney said: “Our members are dedicated to keeping London moving. Now they are being told that they will be poorer in old age. This is an appalling way to treat a loyal and committed workforce.
“Workers are balloting for industrial action as a last resort. Despite repeated calls to management there have been no guarantees on pensions or job cuts.
“Strike action would inevitably cause severe disruption to public transport throughout London.”
Unite’s members at TfL are spread across different parts of the organisation, including Dial-a Ride, London Underground and Croydon trams.
The union said its members are also in dispute over pay and the threat of job losses.
The ballot will close on May 26. If members vote in favour of industrial action, strikes could begin by mid-June, although Unite said action is likely to be co-ordinated with other unions who also have members in TfL.
A TfL spokesman said: “A review of TfL pensions was a condition placed on TfL by government as required as a result of its funding agreement.
“We’ve now received the final report from the independent review into the TfL pension scheme, which has been facilitated by Sir Brendan Barber with the support of pensions expert Joanne Segars.
“The report rightly highlights the complexity of the issues related to pension reform and TfL will now take time to consider next steps.
“There are currently no proposals for changes to TfL’s pension arrangements. Any reform proposed in the future would be subject to consultation with all stakeholders, including staff and trade unions.”