The line is a direct link to the City

Safety chiefs are concerned that loose bolts were found on the four trains on Tuesday after they returned to service following the crash at Chancery Lane on 25 January, which injured 32 people.

The trains were taken out of service as they are the same design as the Central Line train which derailed when a motor is believed to have sheared away from securing bolts and hit the track.

They were modified and put back into service on Tuesday, but a routine inspection by engineers found bolts holding a safety bracket beneath the motors were loose.

It is absolutely ludicrous that they are running these trains when the problem has obviously still not been identified Bobby Law, RMT

Drivers had threatened to refuse to work if London Underground (LU) could not give them assurances that the trains were safe to drive at a meeting on Thursday.

LU is bringing in an independent team of aviation technicians to oversee safety checks, as well as auditors from the nuclear industry to double check the work.

Meanwhile, services on the line were suspended on Thursday morning because of a signal failure, which LU said was not linked to the bolts problem.

Bobby Law, London organiser for the Rail Maritime and Transport Union, wanted a guarantee from managers that no more bolts would come loose.

He said: "It is absolutely ludicrous that they are running these trains when the problem has obviously still not been identified."

An LU spokeswoman said it was "not uncommon" for newly-fitted bolts to need adjustment.

More than 80 trains still need to be checked and adjusted on the Central Line which has been suspended indefinitely.

An interim report into the derailment will be published by LU on Friday.