The RMT has rejected an offer for Tube drivers to voluntarily work a four-day week.
Under the proposals, most drivers would see their working week reduced from 36 hours to 35 hours, but their salary would remain the same.
The union said it was concerned about shift lengths, unacceptable working time arrangements and the possible impact of fatigue on safety.
The union it wanted a 32-hour week over four days, which would see drivers work three fewer hours each week for the same salary.
RMT official Jared Wood said: “TfL need to come forward with changes to their proposals for this dispute to be resolved.”
Claire Mann, TfL’s chief operating officer, said the four day week proposal “allows us to offer train operators an additional day off, whilst at the same time bringing London Underground in line with the working patterns of other train operating companies.”
She added: “The changes would be voluntary, there would be no reduction in contractual hours and those who wish to continue a five-day working week pattern would be able to do so.”
