The hot September sent fashion sales plummeting as retailers failed to sell racks full of autumn and winter clothing, figures show.
Despite the buzz around London Fashion Week, the unexpected heat wave caused chaos for clothing stores.
They recorded a year-on-year drop of 5.9%, beaten only by April's fall of 9.2%, according to the BDO High Street Sales Tracker.
Every week in September saw negative year-on-year sales for fashion, beginning with a drop of 5.5%.
As temperatures topped 34C, fashion sales dropped 8.4% in the third week of the month on the same period in 2015, and 8.1% in the last week.
However, sales of lifestyle goods edged up 0.9% year-on-year and the homewares sector saw growth of 12.6%.
Neither gain was enough to pull the high street up into positive growth, with the month's like-for-like figures dropping by 2.8% - the eighth month in a row with no sign of growth.
Sophie Michael, head of retail and wholesale at BDO LLP, said retailers needed to take the erratic weather as a lesson in the importance of flexibility.
She said: "When you look at individual performances, it doesn't take long to spot who has innovated and adapted their supply chains to be able to cope with unseasonal weather.
"And it's not just the temperature that is in flux. In order to maintain a strong performance, retailers should ensure that they are attuned to the changing demands of consumers and navigating future challenges presented by Brexit.
"Those that can remain flexible, and adapt quickly, will not only stand the best chance of success but will also be well placed to grab the opportunities that will no doubt present themselves in these uncertain times."