Rail bosses are under fire after it was announced the troubled Caledonian Sleeper will delay the launch of its new trains on the Highland service again to support other routes in the central belt.

Serco, which runs the franchise, said they would be diverting the new trains to support the Lowland routes between London, Glasgow and Edinburgh “probably” until September due to high levels of demand.

The new trains, featuring double beds and ensuites and partly funded by the Scottish Government, were due to be rolled out on the Highland service between London and Aberdeen, Inverness and Fort William in the coming weeks.

Caledonian Sleeper said the change would improve the reliability of the lowland service and enable the company to maintain a slightly higher capacity on the Highland route over the holiday period.

They said the delay would also allow the Spanish manufacturer, CAF, time to complete the work required on the new carriages.

Passengers who booked to travel from July 7 can either get a full ticket refund or the fare difference for the old rolling stock reimbursed.

Scottish Liberal Democrat transport spokesman Mike Rumbles told The Herald:

“This perfectly captures the contempt with which everywhere outside the Central Belt is treated when it comes to transportation in Scotland

“This service has been dogged with delays and other problems since its introduction. Now passengers in the north will have to wait months longer for the sleeper trains to reach them.”

Ryan Flaherty, Serco’s managing director at Caledonian Sleeper, said: “We are disappointed to be announcing this further delay, but it is undoubtedly the correct decision and will allow us to improve the reliability and resilience of the Lowlander fleet, and maintain capacity on the Highland route. 

“While we are keen to introduce our new trains on the Highland route as soon as we can, our priority must be delivering a reliable service on the Lowland route and then make sure that each new carriage on the Highland route is ready to welcome guests and deliver a true Caledonian Sleeper experience. “

Serco had previously said that the new trains would be with the Highland services by the end of May.

A  Transport Scotland spokesman called the delay “unacceptable” and blamed the manufacturer for not providing enough trains.

They said: “The introduction of new trains often present significant challenges, however this latest delay is quite simply unacceptable.

“Given we are fast approaching the height of the tourist season, it is disappointing that many customers are as yet unable to fully enjoy the benefits of the new on-board facilities, particularly as bookings have increased and feedback has been positive where the customer experience reaches the standards we expect.

“While the Caledonian Sleeper team are working to correct train faults, the manufacturer CAF’s inability to supply sufficient suitable trains leaves no other option than for service entry to be postponed.

“The Highlander service will continue to run with existing trains until this this is resolved.”

It is yet another setback for the overnight train since the introduction of the new £159 million fleet in April.

The service was launched a year behind schedule and both the south and northbound trains rolled in hours late on the first night.

Technical faults have led to cancellations and breakdowns, leaving customers stranded or locked out of stations after being transported by coach.