Tube passengers endured travel misery today when a signal failure led to severe delays on two lines.
The problem - at Embankment in central London - led to long hold-ups on the Underground's District and Circle lines.
Earlier this week, as many as six Tube lines were hit by delays during the morning rush hour.
Bob Crow, general secretary of the RMT transport union, said: "This is shaping up to be a pre-Olympic London transport week of hell with London mayor Boris Johnson and his officials claiming a brilliant service while millions of Londoners know better.
"We are paying a heavy price for a programme of cuts to staffing and maintenance that has reduced the transport system to total chaos just weeks away from the Olympics and the blame lies firmly with the mayor and his senior officials."
There were also delays today on mainline services run by the CrossCountry and East Midlands Trains companies.
This was due to a signalling problem in the Ambergate area of Derbyshire.
Buses had to replace trains between Matlock and Derby, extending journey times by up to 45 minutes
Trains between Chesterfield and Derby were also diverted.