UPDATE 1-UK's Asos a Christmas winner after stellar sales growth

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LONDON, Jan 25 (Reuters) - British online fashion retailer ASOS on Thursday beat forecasts for sales growth in the key Christmas period, underlining the ability of the online-only players to outmaneuver their traditional rivals.

ASOS's performance, and that of online rival Boohoo , further illustrates how the UK retail sector, and clothing in particular, is being transformed by the internet.

In stark contrast to their stellar numbers Britain's biggest clothing retailer Marks & Spencer this month reported another fall in underlying clothing sales in the Christmas quarter, while rival Next recorded only a small rise.

ASOS, which sells fashion aimed at 20-somethings, said total retail sales rose 30 percent to 790.4 million pounds ($1.13 billion) in the four months to Dec. 31 - ahead of analysts' average forecast of growth of 27.4 percent.

ASOS and Boohoo are winning market share from traditional high street retailers, tapping-in to a generation of consumers who shop on mobile phones and feast on social media.

ASOS said UK sales increased 23 percent to 300.9 million pounds despite "a challenging market," while international sales were up 35 percent to 489.5 million pounds.

The group, which said its performance was driven by its "unique and differentiated product and proposition" said there was no change to its financial guidance for the 2017-18 year.

In October ASOS had increased its sales guidance for the year to growth of 25-30 percent.

Shares in ASOS, which listed at 20 pence in 2001, have increased by a third over the last year.

They closed Wednesday at 6,874 pence, valuing the business at 5.65 billion pounds - some 780 million pounds more than the 134-year old Marks & Spencer. ($1 = 0.7006 pounds) (Reporting by James Davey; editing by Kate Holton)