The threat of a Boxing Day strike by London Underground drivers has drawn closer after an offer to resolve a pay row was rejected.
Aslef said proposals from Tube bosses were "completely unacceptable" and announced a strike ballot.
Drivers will vote in the next few weeks on whether to take industrial action for a second year on Boxing Day, with the result due on December 17.
A union spokesman said: "The proposal is a step back rather than one forward. LU is acting as if they are manufacturing a Boxing Day strike rather than seeking a solution. We will ballot our members as soon as we can."
Howard Collins, LU's chief operating officer, said: "We have been in discussions with Aslef leadership to find a way to end this dispute and have made them what we believe is a very fair offer.
"LU has a long-standing agreement with all trade unions which cover working arrangements on bank holidays, including Boxing Day.
"However, we have shown good faith and tried to find ways forward.
"Last year we reviewed the level of service required for Boxing Day to closely match customer demand. This year we have offered to roster staff on leave and operate the service with staff who volunteer to work on Boxing Day, thereby offering staff greater flexibility when it comes to their leave over Christmas.
"Further industrial action will not achieve anything and I would urge the Aslef leadership to take this offer to their members and work towards resolving this dispute."