The delayed Night Tube has suffered another setback with the Rail, Maritime and Transport union saying a revised pay and conditions offer "falls short" of what it wanted.
Last week London Underground said it would recruit part-time staff to cover nightshifts at weekends.
It also offered to extend the three-year pay offer to four years.
But the RMT said reps had reported that the offer did not meet drivers' expectations.
General secretary Mike Cash said the union's executive committee would now consider its next steps.
Steve Griffiths, chief operating officer of London Underground, said the latest offer was "fair and affordable".
"We have listened to union feedback and have made absolute guarantees which mean no existing driver will have to work the Night Tube, unless they choose to do so. Instead we will hire part-time train drivers specifically to run the service.
"We have also been working hard to secure a long-term pay deal in order to provide certainty for our staff and for London, so we have now added a fourth year to the deal."
London Underground workers held strikes in July and August over the impact the proposed 24-hour service at weekends would have on safety issues and their work-life balance