The Met Office, citing provisional statistics, said September's mean temperature of 15.2 degrees Celsius matched a record figure from 2006, making it the joint-highest for the month since records began in 1884.
"September 2023's temperature was substantially influenced by climate change and our attribution study shows how this figure would have been practically impossible in a climate without human-induced greenhouse gas emissions," Met Office Senior Scientist Jennifer Pirret said in a statement.
"This September's temperatures are still unlikely and needed the right combination of large-scale conditions and weather patterns, but climate change has helped to make it possible."
Last month also had the hottest day so far in 2023, although September was also wetter than usual, with the UK recording almost a third more rain than average.
Climate scientists have said record-breaking temperatures are set to become more common in Britain in coming years.
Met Office statistics show Britain's 10 warmest years on record have all occurred this century, including its hottest ever day - recorded in July last year - fuelling fires and contributing to heat-related deaths.
(Reporting by Sachin Ravikumar; editing by William James, Sarah Young and Christina Fincher)
