London Underground shut down the Northern Line on Wednesday evening after drivers said the trains were too unsafe to drive.
It has not said when the line, which carries more than 660,000 people every weekday, will re-open.
Drivers have refused to take out trains after the failure yesterday of an emergency brake system designed to prevent trains passing red signals. The system has failed four times since 9 September.
“Our members have the legal right to refuse to undertake duties that would put themselves or the traveling public in danger,” said Bob Crow, RMT union secretary.
London Underground believed the problem had been fixed but, during tests on Wednesday a train passed a red light and its emergency brake failed.
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The Northern Line has been running a reduced service for the past two weeks as London Underground and the maintenance companies repaired the trains. Last week it doubled up drivers in each train.
The rail operator will meet with RMT and Aslef unions today in an effort to get drivers to return to work. Aslef has warned that it will ballot members for industrial action if drivers are sent home without pay.
