Commuters endured severe delays on the Northern Line as a result of faulty back-up brake equipment.
The RMT is calling for daily safety checks
London Underground (LU) said for safety reasons, from Thursday Tube cabs are being manned by an extra driver, leading to a reduction in services .
The Rail Maritime and Transport (RMT) union said 20 drivers had refused to work over concerns about the brakes.
The RMT said the equipment, which stops trains if they pass a red light, had failed four times in as many weeks.
London Underground said it was looking for other staff to volunteer to ride in the cabs with drivers "as a fail-safe".
An LU statement assured passengers it was continuing to press Tube Lines and Alstom, the firms responsible for maintaining the line, to resolve the issue.
But it advised them to use alternative Tube, bus and rail services where possible as LU tickets would be accepted.
Two weeks ago LU announced it was checking trains on the Northern Line for problems with back-up brakes.
The RMT is demanding daily safety checks as well as the double-crewing of trains.
Bob Crow, of the RMT, said: "This system is designed to prevent the potentially disastrous consequences of trains passing signals at danger, and the seriousness of these failures cannot be understated."