The 'trip-cock' emergency braking system, designed as a failsafe to prevent trains passing signals at danger, has failed four times in as many weeks and RMT last week insisted on double-crewing, daily brake tests and urgent action to identify faults.

It is now understood that inspection by LUL engineers at the depots of Tubelines' subcontractor Alstom unearthed a number of faults and exposed failure to maintain key components of the emergency braking system.

"We welcome LUL's decision to impose its own emergency direction on the maintenance of these trains, and that, as a result, shortcomings in Alstom's maintenance regime have been exposed." RMT general secretary Bob Crow said today.

"We also welcome the fact that daily brake checks are now being undertaken on all Northern Line trains, that certain modifications are already being made, and that our safety reps are now fully involved in finding a solution to this very serious problem.

"As a result of the emergency measures undertaken we have allowed double-crewing to be withdrawn, but our safety reps will be keeping a very close eye on developments and RMT will not hesitate to insist on its re-introduction should it become necessary.

"The private sector is making £2 million a week out of its contracts on the Tube yet it is not only failing to deliver promised improvements but has also demonstrably failed to maintain a crucial safety system to acceptable standards.

"This emergency has again exposed the sheer folly of the fragmentation of London Underground's infrastructure, and we will continue to campaign for the work to be brought back in-house," Bob Crow said.