The train derailed and hit the tunnel wall at Chancery Lane

The Central Line was suspended for months after a train derailed in January, injuring 32 people.

An independent inquiry team found it crashed after a traction motor beneath the train fell off and hit the track.

Now LU's own technical report, due to be published next week, has discovered the reason it fell off.

It found bearings in the gearbox disintegrated and the shrapnel caused the drive shaft to fail.

The motor then started vibrating in the mountings, which melted away, and fell onto the tracks.

Quick fix

LU has been criticised for not looking more closely at what caused two previous cases of motors falling off trains.

It carried out a bolt inspection on the train's motors, which did not identify faulty bearings as the problem.

Rick Justham, from the transport union Transport Salaried Staffs' Association (TSSA), told BBC London: "Alarm bells should have rung. There were quite clearly problems here in terms of looking for a quick fix.

"What we have to do is make sure all investigations in the future are very thorough and actually look to the root cause, not just skimming over for a quick fix.

In a statement LU said: "We are fully committed to addressing the matters raised in the full report when it is published.

"In the meantime LU, our contractors and the railway inspectorate are satisfied the trains on the Central Line are safe."

The findings may mean designing and fitting 5,500 new gearbox bearings.