The line is known as 'The Drain'

Bolts had to be adjusted on trains within hours of them being restored to service after having their motors checked following an accident.

LU said a "handful" of bolts on the Waterloo and City Line trains needed tightening and announced a review was being carried out on inspection procedures.

But a union official has said drivers may not work on Thursday morning if they feel the trains are not reliable.

The trains were only returned to service at 1330 GMT on Tuesday after being checked in the wake of the derailment at Chancery Lane on 25 January which left 32 people injured.

Tube drivers had asked for assurances about the safety of trains and it is possible they will refuse to drive them in the morning unless they are satisfied RMT's Bobby Law

The Central Line has been suspended since the accident so that train motors can be checked and revamped.

London Underground said engineers had carried out a routine inspection of the trains on Tuesday night when they discovered that a number of bolts had not been adequately tightened.

Bobby Law, the Rail Maritime and Transport Union's London organiser, said he would be raising the issue as a matter of urgency during a meeting with managers on Thursday morning.

"Tube drivers had asked for assurances about the safety of trains and it is possible they will refuse to drive them in the morning unless they are satisfied," Mr Law said.

Preliminary investigation findings of the Chancery Lane accident showed the cause as a motor shearing away from securing bolts, hitting the track and derailing the train as it entered the station.

LU said fatigue was the most likely cause of the bolt failure, compounded by a safety bracket failing.