ScotRail has drafted in a former rail chief to improve punctuality after it failed to hit a target last month despite an improvement plan being in force for more than a year.
However, transport minister Humza Yousaf said he expected the fall in performance "to be addressed immediately".
Figures published today show punctuality over the year to last Saturday was 90.4 per cent of trains arriving within five minutes of schedule, compared to the target of 90.8 per cent.
That was set under a detailed performance improvement plan which has been in force since it was ordered by Mr Yousaf in September 2016 when punctuality fell below minimum acceptable levels.
The target was set lower than the original target for this period, of 91.7 per cent, to make it more achievable.
Former Transpennine Express managing director Nick Donovan has been brought in to carry out an independent review of performance.
The ScotRail Alliance with track owner Network Rail said his initial assessment was expected within weeks, followed by "concrete steps" next month.
The announcement comes two days after Mr Yousaf tweeted that performance "hasn't been to the standard I'd expected" and he was making plans to ensure it improved.
He said today: “I completely understand the frustration of any passenger who has had their journey impacted by poor performance.
"ScotRail has faced a number of challenges, not least the impact of seasonal weather, which affects most UK operators.
"However, this on its own can’t and won’t be used as an excuse.
“It should be acknowledged the existing performance improvement plan had been working well, but the dip in performance since autumn must and will be addressed.
"This should not take away from the very real efforts made by staff up and down the country in the face of unrelenting, often unwarranted, criticism, and their dedication should be commended.
“I am not complacent, and fully expect this recent downward trend to be addressed immediately."
The alliance blamed poor performance over the last month on a "challenging" autumn and winter.
It said major incidents had included Edinburgh-Glasgow signalling work finishing late along with frozen points at Edinburgh Waverley on 27 December, a signalling failure in the Cathcart area of Glasgow on 15 December and a broken down train at Glasgow Central on 11 December.
Punctuality for the four weeks to 6 January fell to 83.4 per cent compared to 89.7 per cent in the same period last year.
The figure for the year to 6 January increased from 90 per cent to 90.4 per cent, but this was not enough to meet its target, which is progressively raised each month.
The equivalent figure was 89.2 per cent when the performance improvement plan started.
Last month was the first time since last May the target has been missed.
However, the alliance said the latest figures meant it was still the best performing large train operator in Britain.
It said Mr Donovan, who has already started work, would "provide additional resource to the expertise already in place across the ScotRail Alliance to propose steps for improvement".
ScotRail Alliance managing director Alex Hynes said: "Our customers deserve a better and more consistent service from us, and we are determined to deliver that."