Labour's Sadiq Khan is facing his first London Underground strike as Mayor of London after maintenance and engineering staff on the network backed a walkout. The RMT union claims Underground bosses refuse to discuss outstanding pension issues relating to the Night Tube until after the launch of the operation.
The long-awaited service will roll out across Central and Victoria lines on 19 August, with the Jubilee, Northern and Piccadilly lines launching late-night services in the autumn. But the RMT warned there are "major" unresolved issues over staffing and remuneration arrangements.
"RMT members have now shown their anger at the way that Tube Lines have attempted to tie in attacks on pensions with the offer on pay and the Night Tube," said Mick Cash, the general secretary of the union.
"There are also major unresolved issues over the Tube Lines staffing arrangements for the Night Tube and over performance-related pay."
The strike ballot will now be considered by the RMT's executive, the union's ruling body, which could announce a walkout date unless the dispute is resolved.
"It is absolutely essential 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," Cash added.
The service, which will run on Friday and Saturday nights, was initially meant to launch in September 2015. However a number of industrial disputes with Underground unions, including the RMT, Aslef, Unite and the TSSA, delayed its roll-out. Transport for London has claimed the Night Tube will create almost 2,000 permanent jobs over its lifetime.
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.
"Pension arrangements for Tube Lines maintenance staff have been the subject of separate ongoing discussions, and we agree that it's important that these talks resume as soon as possible so the dispute can be resolved."