Train services are unlikely to meet punctuality targets until 2021, according to a ScotRail forecast.
A review commissioned by the under-fire operator found there will have to be major improvements to meet performance standards.
Abellio, which runs the franchise, hopes passengers will notice a significant difference by March this year, but does not expect to hit the “public performance measure” for another two years.
The agreed target with the Scottish Government is that 92.5% of services arrive at their final destination within five minutes of the stated time.
Lesley Laird, the Fife MP and shadow Scottish Secretary, said: “In no other part of working life would anyone get away with this.
“The SNP Scottish Government should use the break clause and end this shambles now.”
An internal document for ScotRail shows it expects the PPM, which currently stands at 87.3%, to hit 92.4% by March this year, and then 92.5% by March 2021.
Consultancy firm Nichols conducted the analysis into whether the rail operator is making progress on recommendations made in an independent review published last year.
The Donovan Review, commissioned by the ScotRail Alliance and led by former TransPennine Express managing director Nick Donovan, made 20 recommendations on how ScotRail could improve its quality of service.
The Nichols report stated: “Despite the evidence of significant activity in regard to the implementation of the recommendations since the publication of the Donovan report, there has been a further decline in performance and this raises questions about when the positive effects from implementation of the recommendations will materialise.”
Transport Secretary Michael Matheson has warned Abellio it could be stripped of the ScotRail franchise if performance does not improve.
There have been a number of cancellations across the service following the introduction of a new timetable in December, with delays in acquiring new trains and the subsequent training of staff given as key reasons behind the problems.
A ScotRail Alliance spokesman said the rail operator acknowledges performance has not been good enough.
He said: “We’re really sorry to customers for the recent disruption to services. We know the impact this has on your day and are working flat out to get things back to normal.
“Actions from the Donovan Review have been praised in the Nichols report, and these actions have seen the banning of skip-stopping except as a last resort and improvements in train reliability.
“But our performance is not where we want it to be, and as this year progresses our customers will see improvements.”