The key measure on which the operator is judged has fallen to 89.7 per cent of trains arriving within five minutes of time in the year to 3 March.
Performance against this industry standard was 0.4 percentage points worse than in the year to February.
The figure has also fallen every month since it peaked at 91.2 per cent last August.
Punctuality in February also fell, by 1.2 points to 86.6 per cent, over the four weeks to 3 March.
The further setback for passengers comes weeks after The Scotsman revealed commuters on ten rush-hour services a day on the main Edinburgh-Glasgow are being crammed into fewer carriages because the leases for some trains has run out.
ScotRail also admitted today that around 2 per cent of services on the route - or one in 50 - were cancelled or missed station stops.
It said that was a similar figure to the rest of its network.
A plan to improve performance by former rail chief Nick Donovan, who was drafted in as a troubleshooter, is due to be completed shortly.
However, the ScotRail Alliance with track owner Network Rail said it remained the best-performing large operator in Britain.
ScotRail Alliance operations director Perry Ramsey claimed: “We faced the worst snow this country has faced for generations, which has had an understandable impact on our performance."
He added: "We know how frustrating the disruption was for our customers, and appreciate their patience as we worked to get things back to normal.
“The fact that ScotRail remains the best performing large operator in Britain is down to the hard work of our staff in the ScotRail Alliance.
“The thousands of people in ScotRail and Network Rail Scotland went above and beyond the call of duty to keep people moving where possible, and then to get the full railway back open again as quickly as possible.”
ScotRail also claimed the snowfall on 28 February had triggered "the first ever Met Office red weather warning for snow in Scotland’s history".
In fact, a previous red warning for snow in Scotland was issued by the Met Office in 2010.
The agency changed to using "impact-based" criteria for severe weather warnings the following year, which has led some to believe no such warnings had previously been issued.