Britain's Black Friday expected to be a record-breaking online affair

* Black Friday is on Nov. 25

* UK spending forecast by PwC to jump 38 pct to 2.9 bln stg

* More retailers offering earlier deals this year

By James Davey

LONDON, Nov 21 (Reuters) - Spending on Britain's Black Friday discount shopping day is forecast to break last year's record takings, though it is expected to be predominantly an online affair this year.

Last year marked a change in the nature of the event imported from the United States. It generated record revenue but was unexpectedly subdued in terms of store-based sales, with shoppers put off by bad weather and memories of chaos and scuffles in 2014.

Shoppers searching for deals are expected to focus more sharply on online offers rather than those in stores.

PwC, the accounting and consultancy firm, is forecasting revenue from Black Friday promotions to grow by 38 percent to 2.9 billion pounds ($3.6 billion).

Its research found that 27 percent of British adults intend to buy something over the weekend and on average expect to spend 203 pounds, with 77 percent of the survey's respondents planning online purchases, against only 17 percent setting their sights on in-store deals.

Black Friday, the day after the U.S. Thanksgiving holiday, was so named because spending would surge and retailers would traditionally begin to turn a profit for the year, moving from the red into the black.

A fixture in Britain since 2010, it falls on Nov. 25 this year.

The day has developed into an important part of a Christmas trading season that now has three peaks - Black Friday, the week up to Dec. 25 and then the post-Christmas sales, when products are offered at reduced prices.

Analysts argue that Black Friday discounts pull forward Christmas sales that store groups would otherwise have made at full price and can dampen business in subsequent weeks.

Retailers, however, say that carefully planned targeted promotions with global suppliers allow them to achieve a sales boost while maintaining profit margins.

This year more retailers, including Amazon, Dixons Carphone, Argos and Tesco (Frankfurt: 852647 - news) , have stretched Black Friday promotions over a week or two to smooth demand and reduce pressure on supply and distribution infrastructure.

Dixons Carphone (Frankfurt: CWB.F - news) 's Currys PC World electrical retail chain is offering deals on more than 3,500 products over 10 days. It is anticipating 141 orders per minute over the period, with shopping on mobile devices expected to be greater than that on desktop computers for the first time. Amazon launched its Black Friday sale on Nov. 14.

"Retailers have been, understandably, learning about the pitfalls of over-distributing overly discounted lines to eager shoppers ahead of what has traditionally been peak trading season," said Shore Capital analyst Clive Black.

"Equally, shoppers have sussed out a lot of the specially ordered low-quality tat that comprises Black Friday, too, so conditioning some of their behaviour." ($1 = 0.8091 pounds)

(Editing by David Goodman)