More UK rail passengers are becoming dissatisfied with their train journeys, according to the latest annual survey, with some seeing their timetable as “a work of fiction”.

The research, which polled 27,000 people across Britain at the time of travel, found almost one in five passengers was left discontented, with ever fewer seeing their services as reliable or able to find a seat on board.

The study, by the watchdog Passenger Focus, was conducted in the autumn, before the rail chaos and disruption over Christmas.

Train operators vowed to improve performance after 81% of passengers expressed satisfaction, 2% down on the previous year.

The worst-rated operator was Southeastern. It received a satisfaction score of 74%, three percentage points below Thameslink and Southern, which recently admitted that one of its train services had never run on time for an entire year. All three commuter services are operated by Govia, a joint venture between Go-Ahead and Keolis, a subsidiary of SNCF, the French national railway.

At 93%, Chiltern was the highest-ranked operator of a rail franchise, while the limited services offered by Heathrow Express and Grand Central received a 94% satisfaction score.

Anthony Smith, the chief executive of Passenger Focus, said: “Rail passengers’ satisfaction is driven by getting trains on time. Many are “being let down – fare increases, billions in government investment and promises of improvement don’t seem to be delivering change on the ground.

“The high-profile disruption after Christmas and at London Bridge will only have added to the gloom.

“Passengers do not care who is to blame for things going wrong and for some of them the timetable is a work of fiction. If it really is the case that better day-to-day performance can’t be achieved, then an honest, open debate is now needed so that passengers might be able to trust the promises made by the industry again.”

The number of passengers who said their train was punctual and reliable fell by 2% to 77%. Fewer than half thought fares offered value for money, a similar level to last year. However, station facilities were thought to be improving.

London and south-east England commuters in particular were increasingly critical of how staff assisted them and dealt with delays.

Southeastern said the scores reflected “a difficult autumn period”. David Statham, the train operator’s managing director, said: “We recognise that we have a lot more work to do to ensure that our passengers are more satisfied with the service that we provide.” He said the company was investing more than £70m in the next three years and that passengers would start to see the results in coming months.

Michael Roberts, director of the Rail Delivery Group, which represents train operators and Network Rail, said: “These results show how passenger satisfaction has been dipping after years of steady improvement and they reinforce our determination to do better.

“The timetable is our promise to passengers, and too often we aren’t making good on this commitment. To improve, we are working closer together as an industry to deliver better reliability so that more people can complete their journeys on time and improve information to passengers, particularly during disruption.”

Unions said it was further proof that the rail system wasn’t working. The RMT general secretary, Mick Cash, said: “This report throws up yet more evidence that two decades of privatisation, fragmentation and underinvestment in Britain’s railways have left passengers angry and frustrated as they see the hard cash they spend on extortionate fares drained away in profits to the train operating companies.

“It is telling that satisfaction on the commuter routes into London, where there is a drive to axe guards and station staff in the name of profit, is in freefall.”

Labour’s transport spokesman, Michael Dugher, said: “Since 2010, overall passenger satisfaction with their journey is down, the percentage of passengers satisfied with punctuality is down and the percentage of passengers satisfied with value for money is also down.

“At the same time, people have been hit with massive fare rises of over 20%. The government needs to get a grip. David Cameron’s approach of just sitting back and doing nothing to reform the railways is not an option.”

• This article was amended on 28 January 2015. It wrongly claimed that Southeastern had recently admitted that one of its train services had never run on time for an entire year. The operator with that record was Southern. This has been corrected.