The price tag for modernising a large section of the London Underground has risen by more than £1bn.
The upgrade of the 'sub-surface' lines will also be more than three years later than planned.
The rising cost and delays have been caused in part by the collapse of a crucial contract to replace 'antiquated' signals.
London Underground abandoned a deal with Bombardier in 2013 with an estimated loss of almost £100m.
The modernisation of the Circle, District, Hammersmith & City and Metropolitan Lines will now be completed in 2022 instead of 2018.
The total bill has risen by 30% from £4.26bn to £5.54bn.
London Underground said the commuters would see a significant improvement in services.
The number of trains on the Circle Line is to double with a frequency of one every four minutes instead of every ten minutes.
Labour said the collapse of the Bombardier signalling contract had caused 'expensive chaos'.