Tube maintenance firm Metronet could be fined more than £1m if it is found responsible for Monday's severe delays, Transport for London sources have said.
Overrunning engineering work caused some of Monday's delays
Tens of thousands of commuters returning home were hit by severe delays on the Central and Circle lines.
Mayor Ken Livingstone said such frequent delays were "inconceivable".
He warned Metronet to improve its service by early next year and said if it did not, he would urge the government to reconsider its contract.
Metronet, which is a consortium of contractors working in a private public partnership (PPP) scheme, could face a £1m fine for delays caused by signal failures on the Central Line and another £100,000 penalty for disruptions due to overrunning engineering works on the Circle Line.
Speaking during his weekly press conference Mr Livingstone said that overdue engineering works were not only causing huge financial losses but also affecting London's image.
"Unless there is evidence of a marked improvement it is inconceivable that things can carry on... to say to Londoners there will be another 12 years of this is inconceivable," Mr Livingstone said.
Metronet staff were not punctual and often underperformed, he said.
He also said that passengers should not have to pay for financial gaps incurred by the firm.
Metronet said they were still looking into the severe delays.
"That investigation is to continue until we reach an outcome and decide where the attribution lies in terms of penalties," a spokesman said.
The Rail, Maritime and Transport Union's general secretary Bob Crow said: "We don't want the privateers fined, we want the work brought back in-house.
"What's £1m to a firm that makes more than that in profits every week?"