LONDON (Reuters) - The number of shoppers in Britain visiting stores on the day after Christmas fell again this year, data showed on Thursday, underscoring how what was once a key date for retailers has been supplanted by Black Friday sales.

Footfall on Dec. 26, known in Britain as Boxing Day, was down by an annual 3.1 percent, the third year in a row to show a decline, market research firm Springboard said.

Compared with Black Friday, which fell on Nov. 29, footfall was down 10.7 percent, Springboard said as it updated preliminary figures first published on Wednesday.

Many retailers had a tough 2018 when they were forced into heavy discounts to compete with online shopping and to win over consumers whose earnings, when adjusted for inflation, are lower than they were a decade ago.

The retail sector is also facing uncertainty over Britain’s exit from the European Union, higher labour costs and an increase in business property taxes.

Clothing chains Primark ABF.L, Superdry SDRY.L and online retailer ASOS ASOS.L have warned of weak sales. Sports Direct SPD.L owner Mike Ashley said it was "the worst November in living memory" for retailers.

Springboard said many people were visiting shops as they went out to eat and drink, a trend that was benefiting high streets and other venues more than out-of-town retail parks.

London’s Oxford Street, Regent Street and Bond Street saw a 15 percent increase in footfall year-on-year in the first hours of shopping of Dec. 26, the New West End Company said, boosted by international tourists attracted by the fall in the value of the pound after the 2016 Brexit referendum.