UPDATE 2-Stellar sales growth makes Asos a Christmas retail winner

* Four months retail sales up 30 pct

* UK up 23 pct, international up 35 pct

* Full-year guidance maintained

* Shares down 0.3 pct, hit 52-week high on Wednesday (Adds detail, analyst comment, shares)

LONDON, Jan 25 (Reuters) - British online fashion retailer Asos on Thursday beat forecasts for sales growth at Christmas, underlining the ability of pure internet players to outflank traditional rivals burdened with big store estates.

The performance of Asos and online rival Boohoo shows how the internet is reshaping the British retail landscape and the clothing sector in particular.

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

Established in 2000 for fashion-conscious twenty-somethings, Asos is one of Britain's biggest e-commerce success stories.

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

Asos 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. Retail gross margin was up 80 basis points year-on-year.

Asos shares, which listed at 20 pence in 2001, have increased by a third over the last year and hit a 52-week high on Wednesday, making it one of the most expensive stocks in the sector with a price/earnings (PE) ratio of 90.

It has a market capitalisation of 5.65 billion pounds ($8.06 billion) some 780 million pounds more than the 134-year old Marks & Spencer.

The stock was up 1.3 percent at 6,958 pence at 0905 GMT, with enthusiasm tempered as it did not combine the sales beat with an upgrade to full year guidance.

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

"We are confident Asos can sustain high levels of sales and earnings growth thanks to its large addressable market, deep competitive moats and ongoing innovation," said RBC Europe analyst Sherri Malek.

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.

"We achieved an exceptional performance in the UK, whilst momentum in international sales continued," said Chief Executive Nick Beighton. He said Asos's performance was driven by its "unique and differentiated product and proposition." ($1 = 0.7013 pounds) (Editing by Kate Holton and Keith Weir)