London is facing the prospect of more strike action over the Night Tube.
Members of the RMT union are voting on whether to strike over their pay and conditions under the Night Tube deal.
The ballot for action involves nearly 1,500 maintenance and engineering staff and concerns pay and pensions.
The union agreed a deal for its drivers in March after months of wrangling between Transport for London and the unions.
The latest ballot opened yesterday and will close on Tuesday 24 May.
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 a thousand 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. It is absolutely essential now that there is the earliest possible resumption of serious and meaningful talks on these issues and the union is ready to engage in those talks. In the meantime, the ballot continues.”
Steve Griffiths, chief operating officer of London Underground, said: “We have made the same fair and sustainable four-year pay offer for Tube Lines maintenance staff as has been made for staff employed by London Underground.
“This will see an average two per cent salary increase this year and inflation-protected rises in 2016 and 2017. Pension arrangements for Tube Lines maintenance staff have been the subject of separate ongoing discussions.”