Passengers were initially due to use the new carriages in April, but the company blamed production delays with Spanish-based manufacturer Construcciones y Auxiliar de Ferrocarriles (CAF).
The rollout was postponed until October, but the operator today announced another delay in delivery of the service.
CAF were also responsible for building the trams currently running on Edinburgh’s city network.
In a statement, Serco said 35 of the luxury carriages - which include double beds and en suite showers - were in the “final build stages,” adding they would be “delivered in the coming weeks”.
It added testing and gaining “regulatory approval” for the new carriages was “time-consuming and complex” and blamed this process as the main cause of the delay.
It is understood as many as 1,800 passengers were booked on the fleet’s journeys between October and March, but the company has said customers will be offered a full refund or opportunity to rebook their travel.
Fees for private ensuite double rooms start from £200 per passenger, while single “club” rooms start from £125.
New features include a hotel-style door card entry system, charging points and wi-fi.
Ryan Flaherty, Serco’s Managing Director at Caledonian Sleeper, said: “We are sorry that we will not be able to launch the service this Autumn, and understand that customers who wanted to travel on them in 2018 will be disappointed.
“But with five different accommodation types, as well as on board catering, dining and shower facilities, this is the most complex introduction of new rolling stock ever undertaken in the UK, and we are determined to get it right.”
He added: “The new sleeper carriages are absolutely superb, and will transform the experience of travelling by train between Scotland and England.”
The launch of the Lowland service between London, Glasgow and Edinburgh will go live first, with the Highland service to Fort William, Inverness and Aberdeen coming at a later date.