The Waterloo and City line, which has just two stops, started operating on Tuesday after being suspended for three-and-a-half weeks.

The line was closed after a Central Line train crashed at Chancery Lane station on 25 January.

More than 30 passengers were injured in the accident, caused by a motor sheared away from securing bolts and hit the tracks.

This is a massive programme of work and will take some time Rob Mason, London Underground

Both the Central and Waterloo and City lines were closed because they use the same type of trains.

But while there were only four trains to modify on the smaller line, more than 80 have yet to be checked on the Central Line, the longest on the network.

Rob Mason, of London Underground, said it was not known when even a limited service could start running again on the Central Line.

He said: "This is a massive programme of work and will take some time."

An estimated 600,000 passengers a day were displaced when both lines closed, pushing more commuters onto other lines and prompting calls to delay the introduction of the congestion charge.