The Office of Rail and Road has fined Network Rail £2 million after commuters faced serious delays to train services earlier this year.
"Our investigation has identified important issues that Network Rail, working with operators, needs to address to improve performance for passengers on these routes," said ORR chief executive Richard Price.
"Our analysis shows that the company needs to develop a much better understanding of the impact of timetabling on the reliability of services and on rail users.
"These serious issues have caused severe disruption and frustration for passengers, most notably affecting services at and around London Bridge. ORR is therefore imposing a £2 milliom fine on Network Rail – a decision we did not take lightly. The scale of the delays suffered by passengers was central to our decision to fine. The penalty sends a clear message to the Network Rail board; Network Rail must urgently rectify these errors and deliver the reliability of services that passengers have paid for."
Phil Hufton, managing director of network operations at Network Rail, said: "At the start of this year we had a number of problems that caused passengers disruption and frustration and we apologise for this. Since then we have proactively invested over £11 million to improve performance for Southern and Thameslink passengers.
"This investment, which has seen the introduction of a revised timetable, improved equipment, the deployment of rapid-response maintenance teams at London Bridge as well as new information screens and better passenger information, is paying dividends and passenger service reliability has now improved by almost 12 per cent since January.
"While the nuts and bolts of our infrastructure are the most reliable they've even been, severe congestion caused by record numbers of trains and passengers makes delivering a consistently reliable service a daily challenge for ourselves and the train operators. At London Bridge we are undertaking the biggest and most complex station and track redevelopment ever attempted on Britain's railways – while simultaneously continuing to keep services running.
"As we are now a public sector organisation, the fine must come from within our existing budget and will mean a reallocation of existing resources to pay it."