Caledonian Sleeper is to delay the launch of new trains on the Highland service to support another route.
The trains - which feature carriages with double beds - are now not expected be introduced until September on the route between London and Aberdeen, Inverness and Fort William.
Instead, the carriages will be diverted to support the Lowland service between London, Glasgow and Edinburgh.
Transport Scotland said the delay was "unacceptable" and penalties applied.
It had been due to be rolled out on the Highland route over the coming weeks.
However, Ryan Flaherty, Serco managing director of the Caledonian Sleeper, said: "We are disappointed 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."
The company said passengers who had already booked to travel on a Highland service from 7 July and who had booked one of the new accommodation options, would be refunded the difference in cost.
A spokesman for Scotland's national transport agency, Transport Scotland, said the timing of the delay was significant.
'Correct train faults'
He 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 is resolved."
Caledonian Sleeper said the delay would give CAF, the manufacturer, more time to fulfil its responsibilities and complete work on the remaining new carriages.
The fleet has been part funded by capital grants from Scottish ministers and the UK government.