Hundreds of London Underground maintenance workers are set to be balloted for strikes in a row over pensions and travel facilities, with the result due weeks before the capital's mayoral election.
Members of the Rail Maritime and Transport union on Tube Lines contracts covering the Jubilee, Northern and Piccadilly lines are being urged to back industrial action.
The result will be known on April 16, with the union having to give seven days notice of any action, meaning the first walkouts could take place from April 23 - 11 days before the mayoral election on May 3.
The dispute is over demands for improved pensions and travel concessions.
Tube Lines was taken over by Transport for London (TfL) two years ago as a wholly owned subsidiary but the RMT said staff do not receive the same pensions or free travel facilities as TfL staff.
RMT leader Bob Crow said: "This is a straightforward demand for parity with other Tube staff.
"We have gone through all the negotiating channels on both these issues but Tube Lines has refused to budge.
"Former Metronet employees have been allowed back into the TfL pension fund and now also have the same travel facilities as other LU employees.
"The union remains ready to talk and a strong mandate for industrial action will show Tube Lines and TfL the depth of anger there is over this blatant lack of fairness."
Lee Jones, Tube Lines operations director, said: "We have been discussing this issue with the trade unions for some time and we will continue to try and find a way forward through constructive discussion.
"We would urge the RMT to withdraw its threat of a strike ballot which will not achieve anything."