Thousands of London Underground workers will start voting tomorrow on whether to go on strike in a row over jobs.
The Rail Maritime and Transport union said 800 jobs are under threat and 140 ticket offices lined up for closure, with further cuts being planned.
General secretary Bob Crow said: "Not only have London Underground failed to give us the assurances that jobs would be protected but we are well aware that behind the scenes even heavier attacks on our members, and the services that they provide to the travelling public, are being cooked up.
"London Underground appear to be willing to rip up the safety rule book when it comes to staffing levels. We already have repeated examples of stations being left unstaffed on a regular basis with dire consequences in the event of a major incident, and RMT will not sit back and wait for a tragedy to hit the network before we act."
Howard Collins, London Underground's chief operating officer, said: "We have assured staff and customers that all of our stations will continue to be staffed at all times while trains are operating and all stations with a ticket office will continue to have one.
"However in the same way that people no longer use their bank branch to cash cheques, the Oyster technology rolled out across Transport for London means that only one in 20 journeys involve any interaction at a ticket office window. What we are seeking to do is deploy staff more visibly in stations where they can more easily assist passengers.
"We are committed to delivering these changes with no compulsory redundancies. The safe operation of our network remains at the top of our priorities and these changes will improve our performance still further."
The ballot will close on August 11.