London (CNN Business) Asos is having a nightmare holiday season.

Shares in the UK online fashion retailer plunged 40% on Monday after it warned that weak sales in the crucial month of November would harm its annual profit for 2018.

"The current backdrop of economic uncertainty across many of our major markets together with a weakening in consumer confidence has led to the weakest growth in online clothing sales in recent years," it said in a statement.

Asos ( ASOMY ) was considered one of the few retail bright spots in the United Kingdom, where many sellers have struggled with the transition to online shopping and an economic slowdown caused by uncertainty about Brexit.

The profit warning at Asos, which wiped more than $1 billion off its market value, shows that even savvy online retailers could struggle this holiday season.

Read More