London's transport commissioner has described as a "shambles" work carried out by Tube maintenance firm Metronet.
Mr Hendy said Metronet had "messed up big time"
Peter Hendy said he was "absolutely furious" Londoners had to suffer because of the firm's failure to carry out routine maintenance work on time.
His remarks come as speed restrictions are imposed on four busy Tube lines because of the hot weather.
He blamed Metronet's management and called on corporate shareholders to take action.
Metronet apologised for the disruption but said it was dealing with an ageing infrastructure which was in "serious need of renewal".
A spokesman for the firm said work to make the tracks withstand the hot weather was expected to be completed before August 2006.
"This basic failure to carry out routine work calls into serious question their stewardship and must cause people to question their professional ability," Mr Hendy told the BBC website.
Aside from imposing fines of up to £1m, Mr Hendy said there was little else he could do to force the private firm to improve its performance.
However, he said he wanted to make it clear to Londoners who was responsible for the delays to their journey.
The temporary speed restrictions were in place on sections of the Piccadilly, District, Hammermsith and City and Metropolitan lines.
Metronet is responsible for upgrading and maintaining two-thirds of the Tube network under a 30-year public private partnership (PPP).
It is a consortium between Atkins, Balfour Beatty, Bombardier Transportation, EDF Energy and Thames Water.
The firm is said to have failed to do routine maintenance work to ensure the tracks do not buckle in the heat, which has resulted in the speed restrictions.
"This is because Metronet have not planned and managed their work to take account of the hot weather of which this is the first hot day," Mr Hendy said.
"They have messed up big time."
Transport watchdog London TravelWatch said Metronet also botched engineering work on the Circle, Metropolitan and Hammersmith and City lines, which caused signalling failures.
TravelWatch chairman Brian Cooke said: "Passengers will be amazed that a company that cannot seem to do the work it is contracted to is allowed to carry on with that contract."