Embattled rail operator Southern has suffered its worst customer satisfaction rating in an annual survey.

The company scored just 21% in the sixth edition of the report by consumer group Which?.

Just one star was awarded for punctuality, reliability, seat availability, frequency and value for money, with almost half (46%) of travellers reporting their last journey was delayed.

Southern passengers have suffered months of disruption because of a row between the company and trade unions over the role of guards and driver-only trains.

A spokesman for the operator's parent company, Govia Thameslink Railway (GTR), accepted that the performance was "unacceptable", adding: "We're sincerely sorry."

Southeastern was the second worst performer in the report, with a rating of 31% including one star for seat availability.

This was followed by Thameslink and Great Northern - also owned by GTR - at 32%, including only one star for frequency and the condition of their carriages.

At the top of the table, five-star ratings for reliability and frequency helped put Merseyrail into first place with a score of 72%, followed by Virgin Trains West Coast (69%) and East Midlands Trains (67%).

The research was based on a survey of 2,218 commuters carried out in October and November last year. Southern's results were based on the views of 256 passengers.

London mayor Sadiq Khan claimed the results were "yet more damning evidence that London's rail passengers are being let down by the private train companies".

He added: "It is only by devolving control of London's suburban rail routes to Transport for London that we can finally get a grip of the crisis."

The RMT rail union's general secretary Mick Cash said: "Southern are the absolute worst of a bad bunch when it comes to Britain's rotten, privatised railways."

Which? is campaigning for train companies to "respect passenger rights" and urged the Government to "swiftly bring forward reforms that put passengers first".

Which? campaigns director Vickie Sheriff said: "The whole sector is continually failing passengers. Overcrowding, delays, short trains, carriages in poor condition - many services aren't providing even the basics."

Jacqueline Starr, managing director of customer experience at the Rail Delivery Group, representing train operators, insisted it is "not true that the whole railway is failing passengers".

She added: "Four in five passengers were satisfied with their train journey in the last independent nationwide survey, which asks more than 10 times as many passengers as Which?, but rail companies know they can improve."

A GTR spokesman said: "Our passengers deserve better and, together with Network Rail and its £300 million funding package to improve track signalling and overall performance, we're working hard to improve the service.

"This survey inevitably reflects the significant impact of the wholly unjustified industrial action being taken by Aslef and the RMT. We have made comprehensive offers to resolve this.

"Our service is also hit by the ongoing additional knock-on delays from the London Bridge redevelopment on Southern and Thameslink and we have also had performance issues on Great Northern caused by weather, signal failures in key locations and problems with the ageing trains that we have begun to replace."