A significant improvement in the punctuality of ScotRail trains was demanded today by the Office of Rail and Road (ORR).
The ORR wants 92.5 per cent services to run on time next year - 3 percentage points higher than at present.
The regulator is backing a similar call by the Scottish Government so more passengers arrive on schedule.
It follows the ScotRail Alliance with track owner Network Rail introducing its second improvement plan in 18 months in April in an attempt to turn round faltering performance.
The same target has been set for the whole of Network Rail's next five-year spending period from 2019 to 2024, or control period 6 (CP6).
However, the alliance said it was "unlikely" to achieve that in 2019-20.
The Scotsman understands that it is seen as unrealistic considering current punctuality levels.
Punctuality is measured as the proportion of trains arriving at their destination within five minutes of time - the "public performance measure" (PPM).
The ORR said the alliance had forecast it would reach 91.5 per cent in 2019-20.
ScotRail punctuality was 91.5 per cent last month, but the average over the previous year was only 89.3 per cent.
It last reached 92.5 per cent in a single month last August but has since dipped as low as 83 per cent.
Punctuality over a year peaked at 93 per cent in 2012.
The ORR said: "While we recognise there are some potentially significant risks, we have decided the ScotRail PPM target for the first year of CP6 should be set at the [Scottish Government] target of 92.5 per cent.
"The obligation on Network Rail is to achieve the target to the greatest extent reasonably practicable having regard to all relevant circumstances."
A spokesman told The Scotsman: "If Network Rail don’t achieve this target, we would review to determine if they had done everything reasonable to achieve this.
"If they haven’t, we may investigate in more detail and our board would decide whether a penalty was appropriate.
"We will be consulting on our approach to enforcement later this year."
However, the ORR said it would be unable to make Network Rail guarantee at least one of the two cross-Border routes would be always open, as Scottish ministers have sought.
It said: "Network Rail has identified some challenges in delivering this requirement.
"Its plan acknowledges the importance of the requirement, but highlights potential difficulties on certain dates because of the construction of HS2 and the impact on Caledonian
Sleeper services during Euston [station closures."
The ORR said it would instead require the body "to use reasonable endeavours to plan to keep at least one cross-border route available at all times".
Network Rail declined to say why the punctuality target was not expected to be hit.
However, chief executive Mark Carne said: "There are still some areas of concern that we will need to work with ORR on before it publishes its final determination in October.”
Scottish Liberal Democrats transport spokesman Mike Rumbles said: “Passengers want trains to be reliable and good value for money.
"If we want to rebuild trust in rail, especially through the winter months, more work needs to be done to improve punctuality.
“The ORR has correctly stated performance targets for punctuality should be set at the highest achievable level and should not be rolled back to suit operators."
A spokesman for the Scottish Government's Transport Scotland agency said: "There are a number of areas in the ORR's publication which reflects our high level output specification, including high performance on Scotland's railways and improved journey times for passengers and freight customers.
“However, there are a number of areas which will require further discussion with the ORR, including the availability of cross-border routes and the considerable increase in central costs at a time when we are seeking further empowerment of the Scotland route.
“We will be working closely with the ORR and the wider rail industry to seek to resolve outstanding issues in advance of the final determination in October.”