Nearly one in 10 trains were cancelled or severely delayed over a three-month period on rail networks run by the operator of the troubled Southern franchise, according to new figures.
Some 9.3% of passenger services operated by Govia Thameslink Railway (GTR) in April to June this year were either cancelled or more than 30 minutes late, the Office of Rail and Road (ORR) 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.
The period covered by the figures coincided with weeks of disruption for Southern as a result of a high level of staff sickness and industrial action over changes to the role of conductors.
Members of the Rail, Maritime and Transport (RMT) union picketed train stations on Thursday in the latest of a series of strikes but the dispute remains totally deadlocked.
The ORR figures show the proportion of cancelled or delayed trains on GTR services in April to June jumped by almost three-quarters (72%) compared with January to March 2016, when the number of late or cancelled services was 5.4%.
Virgin East Coast services were the second most disrupted, with 6.5% of trains either cancelled or severely delayed.
Overall, the rolling annual average to June this year is, at 3.3%, the highest since 2004-05 - up from 3.1% on the previous 12-month average to the end of March 2016.
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."
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.
"The period coincided with the beginning of the RMT dispute over our plans to improve customer service on board our trains and throughout the period, we experienced unprecedented levels of sickness amongst our conductors and a reluctance by train crew to work overtime and rest days.
"Because of the uncertain nature of train crew availability, we were being forced to cancel trains on an ad-hoc basis which seriously compromised our ability to run a robust train service on a day-to-day basis. This meant that our passengers were unable to plan their journeys with certainty, and these problems are reflected in the CaSL (cancelled and seriously late) figures.
"On 11 July, we introduced a temporary revised timetable which, although it had fewer trains, gave our passengers a much greater degree of certainty as to which trains were running, and enabled them to plan their journeys with more confidence.
"Since then, on 5 September, we took the first step to returning to the full timetable with the reintroduction of 119 trains. The remainder of the timetable will be restored over the coming weeks."