Tube bosses have until 5pm today to withdraw plans to axe 800 jobs or face possible industrial action.
The Rail Maritime and Transport (RMT) union said it will go ahead with a strike ballot unless London Underground (LU) calls off its redundancy proposals by that time.
The union claims the company is already running stations unmanned or with just one member of staff, risking the safety of customers.
But in an open letter to staff and Tube users today, Howard Collins, chief operating officer at LU, insisted the firm was committed to customer service.
He vowed there would always be staff present at every station to help passengers, adding that all stations which currently have a ticket office service will continue to have one.
He also stressed LU's pledge that there will be no compulsory redundancies.
Mr Collins wrote: "Today, only one in 20 journeys starts with a visit to the ticket office and this number continues to decline - 80% of all Tube journeys are now made using Oyster.
"So we don't need or want our staff to be stuck behind glass in under-used ticket offices; we want them out on our stations where our customers need them - on platforms, in ticket halls and at gate lines."
But yesterday, RMT general secretary Bob Crow said the axing of jobs would leave stations at a "dangerously low" staffing level.
He said: "We have already warned that the cuts that are being planned by Transport for London will turn Tube stations into a muggers' paradise and it now appears that the company is speeding up the process and is already leaving stations unstaffed or with just one member of staff without any consultation.
"That is a scandalous dereliction of duty and it is only a matter of time now before TfL and Mayor Boris Johnson have a tragedy on their hands as a result of their cavalier disregard for public safety.
"RMT has made it clear right from the outset that we will not sit idly by while the Mayor and his transport officials drive down Tube staffing levels to dangerously low levels. If we have to ballot for action in the interests of Tube safety then that is exactly what we will do."
LU has pledged to work with the unions to achieve the cuts through voluntary redundancy.