Rail punctuality dipped last month as train companies battled floods and high winds.
A total of 83.7% of trains ran on time in the four-week period from December 8 2013 to January 4 2014, Network Rail said.
This compared with a figure of 88.3% for the same four-week period in 2012/13.
Worst-performing company in the most recent four-week period was Southern, where many fallen trees, as well as landslips on the Brighton line, meant that the company was only able to run 73.1% of trains on time.
The next worst-performing company in December 2013 was First Capital Connect (75.9% of trains on time) followed by East Coast (76.9%), Southeastern (77.1%) and South West Trains (78.1%).
Best-performing company in December 2013 was London Overground (96.1% of trains on time) followed by c2c (93.6%) and Merseyrail (93.4%).
Delays attributable to NR - including external factors such as fatalities, which made up around 20% of all delays - accounted for 60% of delays nationally during the period.
NR said: "A succession of tumultuous storms tested the rail industry's resilience during a very difficult period.
"During this period, the railway dealt with multiple and sustained extreme weather conditions. Storm-force winds brought down almost 400 trees on to the railway while torrential rain caused almost 130 floods and 29 landslips, some blocking major routes.
"Thousands of railway staff responded with dedication and professionalism to repair the damage caused."
NR added that "despite these considerable challenges" it had, over the same period, successfully delivered a £110 million investment programme of improvement work involving many engineering projects - the largest ever undertaken over a two-week period.
Southern's Operations Director, David Scorey said: "Despite this poor punctuality result, we are proud of the dedication and effort shown by Southern and Network Rail staff who worked through extremely challenging circumstances to keep people moving throughout this exceptionally difficult period."