Thousands more Tube workers are to be balloted on strikes in a continuing row over industrial relations.
A total of 1,500 train drivers have already voted on the issue with the result due on Thursday.
A further 5,000 workers, including signallers and station staff, will now be joining the vote, the RMT union said.
The RMT claims London Underground has undermined industrial relations “right across the company”.
London Underground said the company was committed to consultation with its employees and trade unions “using our agreed procedures”.
Train drivers belonging to the RMT and Aslef unions have been voting over issues including policy on signals passed at danger, health and safety, harassment and discipline.