(Reuters) - Britain's high streets, malls and online sites were awash with discounts on Friday as more retailers than ever embraced U.S.-style "Black Friday" promotions, seeking to kickstart trading in the key Christmas period.

In the United States the Friday following the Thanksgiving Day holiday is called Black Friday because spending usually surges and indicates the point at which American retailers begin to turn a profit for the year, or go "into the black".

Though Amazon introduced Black Friday to Britain in 2010, last year marked the first time major UK store groups such as John Lewis [JLP.UL], Dixons and Wal-Mart's Asda participated in a serious way.

A survey commissioned by Barclays found that 65 percent of Britain's multi-channel retailers planned Black Friday promotions this year.

This year new participants include Sainsbury's, Britain's No. 3 grocer, which is offering cut price deals on 13 product lines, including TVs, tablets, audio products and kitchen electricals in 485 stores.

Tesco, Britain's biggest retailer, has extended Black Friday promotions to selected stores, having only participated online last year.

Marks & Spencer, Britain's biggest clothing retailer by sales, is also doing more this year, with all online deals, such as 30 percent off Autograph lingerie and men's coats, mirrored in store. Dixons and Argos have also ramped-up their offers.

Asda said its event this year will be three times bigger than last year's, with 441 stores taking part.

British retail sales growth slowed slightly in November against a backdrop of smaller price rises, but retailers are upbeat about Christmas, figures from the Confederation of British Industry showed on Wednesday.

Whether embracing Black Friday makes commercial sense for UK retailers remains open to debate. Analysts say it can delay autumn sales, pull forward Christmas sales that store groups would otherwise have made at full price, can blunt sales in subsequent weeks and also leaves consumers expecting more pre-Christmas promotions.

Some retailers have also been accused of inflating prices before then slashing them to give the appearance of bargains.

(Reporting by James Davey; editing by Keiron Henderson)