RMT also said it would give complete support to drivers exercising their right to refuse to move Northern Line trains on safety grounds

The emergency 'trip-cock' braking system, designed as a 'failsafe' which triggers automatically if a train passes a signal at danger, has failed on four occasions since September 9, the latest last night (Wednesday) at around 23:00.

"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," RMT general secretary Bob Crow said today.

"After the September 9 incident double-crewing of trains was introduced, but this was withdrawn after just one day. Since then, and despite checks on the entire fleet, there have now been three more failures.

"Before last night's incident we had asked for every train's system to be tripped before it comes into service, but LUL has so far refused.

"After last night's failure we are no longer asking but insisting that all Northern Line trains are double-crewed and that each train is checked daily before coming into service until the problem is identified and eliminated.

"The disruption to services that these emergency measures will cause is preferable to running the risk of a potential disaster - it is as simple as that.

"RMT will support any driver who refuses to move a Northern Line train on safety grounds

"Under the PPP it is LUL who run the trains, Tubelines who are responsible for maintaining them, but Alstom who actually do the work.

"That is crazy enough, but under their contract Alstom have 65 days to find the fault. In such serious circumstances that is a complete nonsense, and if Alstom cannot find and rectify the fault the work should simply be brought back in-house," Bob Crow said.