The warning from ASOS, whose shares fell close to a four-year low, showed that even previously high-flying online-only clothing retailers were not immune to a deterioration in consumer sentiment.
Stocks in other British clothing groups fell on fears of poor trading before Christmas when they do much of their business.
Marks & Spencer was down 2.8 percent, Next slipped 3.4 percent, Debenhams lost 6.5 percent and Primark owner AB Foods shed 2.2 percent.
Shares in direct rival Boohoo fell 11 percent, even though it said it was trading in line with expectations, while Germany-listed Zalando was down 14 percent.
Britain's retailers had been hoping Christmas would revive spending after a torrid year for much of the sector that has seen a string of store groups go out of business or announce shop closures.
But with the sector battling subdued consumer spending, uncertainty over Britain's exit from the European Union, rising labour costs and higher business property taxes, as well as unseasonably warm weather, analysts predict few winners over the festive season.
"There's more than Brexit going on here, there is a weakening in consumer confidence and some of that has got good economic reasons behind it," ASOS Chief Executive Nick Beighton told reporters.
ASOS, which targets 20-somethings, emphasised a high level of discounting and promotional activity across the market and also added that conditions in Germany and France had become more challenging.
STORM CLOUDS GATHER
Payments company Visa said on Monday British consumer spending fell by the most since July last month, while another survey said British households' confidence in their finances hit a six-month low in December.
"We knew the high street was struggling due to structural shifts, but ASOS slashing guidance suggests things are even worse in the run-up to Christmas than previously thought for the sector and the strife extends well beyond the high street," said Markets.com chief market analyst Neil Wilson.
The gloom in the run-up to Christmas has been building in Britain.
Last week sportswear retailer Sports Direct said November trading was "unbelievably bad", while clothing group Bonmarche said it was faring much worse than during the financial crisis. Clothing chains Primark and Superdry have also warned of weak sales.
ASOS cut its sales growth forecast for the 2018-19 year to 15 percent from 20-25 percent previously and its earnings before interest and tax (EBIT) margin target for the year to around 2 percent from 4 percent.
It also reduced its planned capital expenditure to 200 million pounds from 230-250 million pounds previously, delaying investment in automation at its U.S. distribution centre by six months.
Analysts at Peel Hunt cut their pretax profit forecast for 2018-19 by 53 percent to 56 million pounds.
"Whilst trading in September and October was broadly in line with our expectations, November, a very material month for us from both a sales and cash margin perspective, was significantly behind expectations," ASOS said, adding that "conditions remain challenging".
ASOS said its sales rose 14 percent to 656 million pounds in the three months to Nov. 30, but its retail gross margin fell 160 basis points as it cut prices and promoted to shift stock.
Sales had increased by 26 percent in its 2017-18 year.
"We are taking all appropriate actions and our ambitions for ASOS have not changed," Beighton said, telling reporters "this is just a bump on the road."
($1 = 0.7945 pounds)
(Reporting by James Davey and Sarah Young; editing by Jason Neely and Keith Weir)
By James Davey