Hundreds of Tube workers are to be balloted for strikes in a row over the treatment of two union members.
The Rail Maritime and Transport (RMT) union has claimed a health and safety representative was sacked because of his role as a union official.
It added that a Tube driver was facing "trumped up" charges as a result of recent strikes over job losses.
Transport for London said it was "irresponsible" to hold the ballot as disciplinary processes were ongoing.
Members of the RMT and the Transport Salaried Staffs Association have staged three strikes in recent months over the threat to cut about 800 jobs.
They are planning another 24-hour walkout from the evening of 28 November.
The RMT general secretary Bob Crow said: "Instead of harassing our members and activists on trumped up charges the London underground management should be directing their energies into reaching a settlement to the on-going disputes over Tube safety and safe staffing levels."
Voting on whether to take industrial action in support of the two men will end on 7 December, raising the threat of industrial action in the run-up to Christmas.
A Transport for London (TfL) spokesperson called for the "pointless and unnecessary" ballot to be cancelled.
In a statement, TfL said: "It is completely irresponsible for the RMT to call a ballot for strike action when the disciplinary and appeal process for both drivers has not been exhausted and in the case of the Northern line driver has not even started.
"For the RMT leadership to threaten to disrupt Londoners in this way shows a complete disregard for Tube passengers."
Talks aimed at averting the next strike will resume on Monday at the conciliation service Acas.
The unions have signalled they would suspend action over the festive period in the row over job losses if progress was made during long-running talks, but there has been no sign of a breakthrough.