They admitted “standards we set for room presentation were not met” after a passenger twice found berths in a “disgusting” state on two journeys within days of each other.

The latest problems with the service are yet another reputational blow to operator Serco, which has suffered a catalogue of faults with its £150 million fleet since being introduced in April.

Several of the trains between Edinburgh, Glasgow and London have broken down, forcing passengers off in the middle of the night, and toilets and showers have been faulty.

Caledonian Sleeper has apologised for repeated cleaning failures. Picture: Photo by Jeff Holmes/Shutterstock

Staff shortages have left passengers unable to get food in the club car, while staff have voted overwhelmingly for industrial action over “intolerable” working pressures.

Introduction of the trains on three Highland routes has been postponed from June until at least September.

Angry passenger Fergus McCallum said he was fed-up with the state of the rooms after his most recent trips from Glasgow to London last Thursday and Sunday nights.

He claimed that bedding had not even been changed, which Serco has denied.

Mr McCallum said: “You are not cleaning the rooms between guests. It’s disgusting, it’s a disgrace.”

He said of Sunday’s journey: “This is about the third or fourth time on the new trains I have been given rooms that have simply had the bedclothes used by the previous guests straightened and not cleaned, and the toilets and bins and carpets presented as left by them.”

He said the room was “filthy, bins not emptied, hair on the toilet bowl, toilet roll pieces on the floor... used ear plugs on the bed, children’s sweet wrappers and jewellery, and, as every time, hair on the pillows and beds.

“They are absolutely not cleaning the beds between customers – it’s filthy.”

On Monday he said: “Yet again I am being expected to use a bed that’s not been changed from the previous guest..”

Passenger watchdog Transport Focus urged travellers to claim compensation.

Robert Samson, its senior stakeholder manager in Scotland, said: “Passengers continue to face a significant level of unpredictable delay on the Caledonian Sleeper.

“Anecdotal reports also suggest some face significant problems that result in a poor journey experience.

“This partly explains why satisfaction levels with the service have fallen steadily since February, though these measures also reflect the ongoing disappointment caused by the delayed arrival of long-promised new trains.

“It is vital Serco makes every passenger on a late service aware of their rights to a refund under the delay repay arrangements. The operator must also make it easy to claim and process claims promptly.

“It is equally important passengers express their dissatisfaction by claiming compensation – every time they are eligible – and complaining about poor service quality too.”

Graham Kelly, Serco’s guest experience director for Caledonian Sleeper, has apologised for both incidents.

“We’re sorry the standards we set for room presentation were not met,” he said. “I can reassure guests that beds are changed and rooms are cleaned each day.

“We will be making it clear to our cleaning supplier that this room did not meet the expected standard.”

Caledonian Sleeper admits failings after complaints of ‘filthy, disgusting’ cabins