New figures show 9.3% trains were cancelled or severely delayed over three months on networks run by the troubled Southern franchise operator Govia Thameslink Railway.
Nearly one in 10 trains were either cancelled or more than 30 minutes late in April to June this year, the Office of Rail and Road data showed.
GTR, which also runs the Thameslink, Gatwick Express and Great Northern franchises, was the worst-performing rail operator for the period and the figure was also the highest recorded for the group since current records began in 2004.
GTR: services in April to June jumped by 72% compared with January to March 2016, when the number of late or cancelled services was 5.4%
Virgin East Coast: the second most disrupted services, with 6.5% of trains either cancelled or severely delayed
A GTR spokesman said industrial action and staffing issues had caused much of the disruption and called on the RMT not to "remain part of the problem".
He said: "We fully recognise that these figures are unacceptable and we apologise to our passengers for this."
A spokesman for the Rail Delivery Group, which represents the rail industry, said companies were implementing a £50 billion package of improvements to the rail network to improve reliability long-term.
He said: "This work is causing disruption, including industrial action over changes that will deliver a better service for passengers without compromising safety.
"Nobody wants to see delays but the current disruption will improve in the long run as we deliver the railway the country needs and passengers want."