Union officials threatening to call a Tube strike over Christmas have described an offer of a 35-hour week as a "smoke screen".
The dispute centres on pensions and jobs
Tube maintenance firm Metronet made the offer in an attempt to avert a strike over the festive period.
But the Rail Maritime and Transport Union (RMT) said it would press ahead with a ballot of its 1,800 members.
Officials said the offer was unrelated to the ballot which is over pensions, redundancies and subcontracting.
Metronet said the action was "completely unwarranted" and would cause huge disruption in the run-up to Christmas if it went ahead.
A spokesman for the firm said the offer of Friday afternoons off, which is effectively a 35-hour week, was a "very reasonable deal".
"It is clear the RMT are intent on disrupting the public in the run-up to Christmas," said a Metronet spokesman. "Their ballot of staff is completely unwarranted."
RMT officials said they would continue talks with Metronet over the key issues in the dispute, but the ballot would still go ahead.
Bobby Law, RMT London regional organiser, said: "Metronet's talk about the 35-hour week is a smoke screen, but even so we have rejected nothing out of hand."
The dispute centres on Metronet's plans to subcontract work, make frontline staff redundant and scrap final salary pensions for new recruits.
The firm has announced up to 285 redundancies, but stressed that more than half would be agency staff and the rest would be voluntary redundancies.