Thousands of London Underground workers are to be balloted for strikes in protest at job cuts and Tube ticket office closures.
Members of the Rail Maritime and Transport (RMT) union will vote in the coming weeks on whether to launch a campaign of industrial action in the New Year.
Voting will end in January and the union will have to give seven days notice of action if there is support for stoppages.
The announcement came after London mayor Boris Johnson unveiled huge changes on the Tube, including a new 24-hour service at weekends and staff based in ticket halls and on platforms rather than in ticket offices.
There will be 750 job losses, although London Underground (LU) said it would seek to avoid compulsory redundancies.
LU employs 18,000 people, including 5,500 station staff.
The plans will deliver savings of around £50 million a year, or £270 million over the term of Transport for London's business plan to 2020/21.
RMT general secretary Bob Crow said: "RMT members have now had a chance to study in detail the proposals put forward last week by London Underground which would axe almost 1,000 staff jobs and close all ticket offices across the network. RMT has also now had a chance to thoroughly examine the risk assessment documents passed to the union by the Tube management.
"As a result, it is absolutely clear that the attack on staffing levels and passenger services would have a devastating impact on Tube safety, with assaults and thefts soaring through the roof. It is also clear that the most vulnerable members of our society would be most at risk when it comes to both violence and access to Tube services. The plans rip up promises on ticket offices that the mayor gave to London before he was elected.
"As a result, RMT can confirm that the union will be serving notice today for a ballot for both strike action and action short of a strike with the ballot closing in January. All Underground members will be balloted, as it is crystal clear that every single member of staff will feel the impact from the proposals that have been put forward.
"RMT will work with the communities in the front line of the cuts plans, and with our sister unions, to fight the Tube cuts through combined political, public and industrial campaigning designed to stop these lethal proposals in their tracks."
The mayor said there would be a "proper period of consultation" with staff over the changes, adding that he hoped employees would see the benefit of the proposals.
"I very much hope it will not come to a strike," said the mayor.
LU said only a small fraction of travellers use ticket offices.
Phil Hufton, London Underground's chief operating officer, said: "We've promised customers that we'll introduce a 24-hour Tube service on five lines during 2015, and our commitment is that all Tube stations will remain staffed at all times when services are operating.
"In future there'll be more staff in ticket halls and on gatelines to help customers buy the right ticket and keep them safe and secure.
"We're clear that there'll be a job for everyone at LU who wants to work for us and be flexible, that we'll make these changes with no compulsory redundancies, and that we'll involve staff in our plans at every stage and support them through change.
"We are now consulting with our unions and staff on our future vision, and I would urge the RMT and TSSA to work with us to shape our plans, rather than threaten hard-working Londoners with completely unnecessary strike action."