There’s fresh evidence from the FTSE 100 today of quite how vulnerable retailers are to unpredictable weather.

High street bellwether Next Plc and middle-class favorite Marks & Spencer Group Plc saw their share prices tumble, knocking more than £500 million ($610 million) off the companies’ market capitalizations as analysts voiced concern over the impact of unseasonably warm weather.

Sneaker vendor JD Sports Fashion Plc and Primark owner Associated British Foods Plc also dropped while the FTSE 350 Retailers Index fell to a three-month low.

“It is well understood that unseasonably warm weather through September and October so far is unhelpful for clothing retailers as they launch autumn / winter product,” JPMorgan Chase & Co. analyst Georgina Johanan wrote in a note to clients Monday.

Britain’s autumn / winter selling period is relatively short and a weak start to the season can lead to early discounting, she wrote, placing Next and AB Foods on negative catalyst watch. Next and M&S shares fell as much as 4% on Monday.

After hot summer weather spurred stronger retail sales, high street fashion brands are now struggling with the warmest October in years. Consumers are once again donning summer dresses and sandals rather than splashing out on expensive overcoats, knitwear and boots.

Last month Next raised its profit forecast for the third time in a row as inflation-linked pay rises and better weather at the start of the summer prompted people to buy more clothes. AB Foods is expecting Primark to be more profitable next year while M&S has said its clothing division is gaining market share.

The pressure is now on retailers as the crucial Christmas period approaches. JPMorgan’s Johanan said there are “no jingle bells yet, just warning bells.”

--With assistance from Kwaku Gyasi, Paul Jarvis and Joe Easton.