The Night Tube service was due to begin on some lines in September, but was delayed amid fallouts over staff pay and conditions.
Further disruption has been threatened with 1,300 maintenance workers set to vote on possible industrial action.
London Underground (LU) train and station staff walked out twice in 2015.
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The union has a mandate to order thousands of members out on strike over the issue, and the results of the latest ballot should be announced at the end of January.
The new staff to be balloted work for Tube Lines, a wholly owned subsidiary company of LU.
They are responsible for maintenance and upgrade work on the Piccadilly, Northern and Jubilee lines.
Mick Cash, leader of the RMT, said: "Our Tube Lines members have been offered the same deal on pay and Night Tube as their own LU colleagues and our reps have clearly rejected it as wholly unsatisfactory.
"Now we move to a ballot of our members for both strike action and action short of a strike."
Steve Griffiths, LU's chief operating officer, urged RMT to resolve the issue.
He 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 2% and inflation-protected rises in 2016 and 2017."
More workers will be balloted for strike action in the Night Tube dispute, the Rail, Maritime and Transport (RMT) union has said.