The rate of complaints by passengers travelling on trains operated by Southern Railway's parent company soared at the end of last year.
Govia Thameslink Railway (GTR) received 32.1 complaints per 100,000 journeys between October and December, according to the Office of Rail and Road (ORR).
This is more than four times higher than during the same period in 2015.
Southern has been embroiled in a long-running row with trade unions over the use of driver-only operated trains.
The national complaints rate was broadly stable, although concern over ticket-buying facilities rose by five percentage points.
Liberal Democrat transport spokeswoman Jenny Randerson said: "The service delivered by some operators is something that would damn a developing country.
"In any other job, in any other industry, the bosses who have presided over months and months of misery would be held to account yet these bosses aren't.
"Anyone who uses services like Thameslink is familiar with the constant overcrowding and delays which plague the service.
"The Government should take responsibility and should have stepped in many months ago to stop this chaos."
A GTR spokesman said: "These figures are disappointing but not surprising. This time last year, our complaint rate was one of the lowest in the industry.
"Since then, the onset of industrial action led to a sharp rise in the number of complaints. Now, as the performance of our train service improves, complaint rates are falling once again."
GTR passengers made 19% of all rail complaints in Britain.
The company operates 15% of all services, consisting of Southern Railway, Thameslink, Great Northern and Gatwick Express.