Around 1,500 maintenance workers are to start voting on whether to go on strike over plans for a night Tube in London.
Members of the Rail, Maritime and Transport (RMT) union are being balloted in the next three weeks on whether to launch a campaign of industrial action.
The union said the dispute, affecting maintenance and engineering workers employed by Tube Lines, was over pay, pensions and staffing.
The union accused the company of refusing to discuss pensions issues until after the eventual launch of the night Tube service.
There are also said to be major unresolved issues over staffing arrangements and a threat to pay.
The night Tube was due to start last year but has been delayed because of failure to reach agreement with the unions. The service is now expected to be launched later in the summer.
RMT general secretary Mick Cash said: "RMT members are furious at the cynical way that Tube Lines have attempted to tie in attacks on pensions with the offer on pay and night Tube, and our reps for this group of over 1,000 safety-critical staff have unanimously thrown the whole package out.
"As a result, the ballot for both strike action and action short of a strike opens today.
"There are also major unresolved issues over the Tube Lines staffing arrangements for the night Tube."