Rush-hour commuters are expected to face disruption to their journeys because Tube workers refuse to drive trains over safety concerns.
Union members say they won't drive trains until safety checks are made
Union members are refusing to drive trains on the Central Line after two faults were found on trains on Tuesday.
Aslef and the Rail, Maritime and Transport (RMT) Unions have threatened to strike if any members are disciplined over their refusal to work.
Transport for London (TfL) refused to comment on the issue.
Up to 900 people got off the Central Line train at Holland Park when it stopped part-way out of the west London station at 0900 BST on Tuesday.
A joint comment from Aslef and the RMT said: "If any of our members are sent home for refusing to move trains for safety concerns then we will ballot for strike action."
Both unions are demanding that engineers inspect the entire fleet of trains on the Central Line before their members go back to work.
But TfL refused to comment on the union's comments over safety.