LONDON, Oct 14 (Reuters) - British online fashion retailer ASOS on Wednesday reported a quadrupling in full-year profit, benefiting from strong demand during the COVID-19 pandemic and forecast more improvement in the 2020-21 year.

ASOS, whose fast fashions are popular with shoppers in their twenties, made a pretax profit of 142.1 million pounds ($183.4 million) in the year to Aug. 31 - in line with guidance in August and up from 33.1 million pounds in the same period last year.

Sales rose 19% to 3.26 billion pounds as it served 23.4 million customers, up 3.1 million.

ASOS traded through coronavius lockdowns when store-based rivals had to close shops for weeks.

The group said it had made a "solid" start to the new financial year and was well set up for the peak Christmas trading period.

However, it said it was cautious on the outlook for consumer demand whilst the economic prospects and lifestyles of 20-somethings remain disrupted.

"Whilst life for our 20-something customers is unlikely to return to normal for quite some time, ASOS will continue to engage, respond and adapt as one of the few truly global leaders in online fashion retail," said CEO Nick Beighton.

Shares in ASOS, which have doubled over the last year, closed Tuesday at 5,378 pence, valuing the business at 5.4 billion pounds - more than twice the market capitalisation of Marks & Spencer, Britain's biggest clothing retailer by sales.

($1 = 0.7747 pounds) (Reporting by James Davey, Editing by Paul Sandle and Sarah Young)