The RMT union has extended its strike mandate on the Tube for six months raising the prospect of possible future disruption for commuters.
Workers voted to keep the strike mandate for another six months in the long-running dispute over jobs, pensions and conditions.
The Rail, Maritime and Transport union (RMT) said its members on the Tube backed continuing with action by 95% on a 54% turnout.
The union said it would now consider its next steps in the campaign.
RMT general secretary Mick Lynch said: "I congratulate our members on delivering another impressive Yes vote in our campaign on pensions, jobs and working conditions on London Underground.
"Our union will now consider the results and the next steps in this campaign."
Members of the union separately walked out on the Docklands Light Railway for 48 hours on Tuesday over pay.
The union has repeatedly brought the London Underground throughout this year with industrial action.
It had planned to take strike action last month, but this was suspended after “significant progress” over job changes and changes to rosters.
However, wider negotiations are still continuing over pensions and working agreements.
TfL has said it believes any changes would create a “fairer, more efficient” London Underground and “give an even better and more reliable service” to passengers.
It comes as the union reached a possible breakthrough with mainline rail operators, which could possibly avoid strike-related disruption at Christmas like last year.
The union, which represents 20,000 crew and station staff, has drawn up a “memorandum of understanding” (MOU) with train operating companies.
In a joint statement on Wednesday, the rail operating companies and the union said that if members accepted the memorandum, it would “terminate the national dispute mandate, creating a pause and respite from industrial action over the Christmas period.”
The statement added: “These discussions would be aimed at addressing the companies’ proposals on the changing needs and expectations of passengers as well as unlocking further increases for staff, in order to help to secure a sustainable, long-term future for the railway and all those who work on it.”
The RMT, as well as drivers’ union Aslef, have staged intermittent strikes since June last year.