Fewer than half of Northern rail services ran on time over the last month, the firm's latest figures have shown.
Just 45.4% of its trains arrived within a minute of their scheduled times between 11 October to 8 November.
Outright cancellations were also at their highest level since July and August.
A spokesman for Northern apologised and blamed "extreme weather" and rail infrastructure.
Between 15 September and 12 October, Northern's figures showed 53.5% of Northern trains were on time, compared to 62.7% of services across the country.
The latest data for Northern during October and November marks a significant fall since the summer, when 61% of Northern services arrived within a minute, matching the figures for the rest of the country.
Cancellations for Northern are now at the highest level since late summer, with 5.2% of services abandoned, compared to 4.4% between July and August.
Pressure group Railfuture said the news came as "no surprise".
"What makes matters worse is when Northern cancels service it is often on the routes where there is only one train an hour, which means you are even later," a spokesman said.
The Northern spokesman said: "We have delivered on our commitment to increase the number of train services, and yet there has been no real increase in track capacity since 2016, which has led to more congested railways across the north.
"This congestion has taken its toll on punctuality. This year has also seen a number of extreme weather events, including unprecedented levels of flooding."
Last month, Transport Secretary Grant Shapps said Northern's current performance "cannot continue", adding the company could be brought back into public ownership if its problems persist.
In 2018, the operator was nicknamed "Northern Fail" by disgruntled passengers when a new timetable led to cancellations, delays, overcrowding and late-running services - causing what a government report described as "significant disruption".
It led to the introduction of an emergency timetable which saw 6% of services cancelled.