The threat of Tube travel chaos has increased after members of the RMT transport union voted overwhelmingly for industrial action in a row over jobs and safety.
Of those taking part in a ballot, 76% voted for strike action and 88% for action short of a strike.
The RMT will now liaise with sister transport union TSSA - which will announce its own ballot result next week - on the next course of action.
Both unions are upset at what they fear could be the loss of 800 jobs and the scrapping of around 140 ticket offices on London Underground (LU).
RMT general secretary Bob Crow said: "Less than two weeks after the Potters Bar (rail crash) inquest delivered the damning verdict that cuts to maintenance works, staffing and inspection frequencies create the lethal conditions that lead to avoidable loss of life, LU management have clearly learned nothing and are burning up safety agreements almost by the day in the dash for cuts.
"RMT members have sent a clear message in this ballot that they will not sit by while the tracks are turned into a death trap and our Tube stations and platforms are left unstaffed and at the mercy of muggers, vandals and other criminal elements."
Mr Crow said London Mayor Boris Johnson "cannot wash his hands of the crisis that is unfolding at LU on his watch".
He continued: "While his managers are ripping up the safety regulations, the Mayor himself is ripping up his promises to Londoners on safe staffing levels and no amount of bluster can get him off that hook.
"We will now use this mandate for action to build up a campaign of industrial, political and public pressure to block the all-out assault on Tube jobs and safety."