The growing trend of UK retailers offering "Black Friday" and "Cyber Monday" discounts could cost the British economy a staggering £117m, according to ELAS.

The law firm, which analysed data from the Office for National Statistics (ONS), warned that UK companies could lose out because of lost productivity as employees rush to grab pre-Christmas bargains during working hours.

"Black Friday" and "Cyber Monday" are traditionally post-Thanksgiving events, but they are becoming more common across the Atlantic as businesses, such as Asda, John Lewis and Argos are putting on offers in Britain.

"Unlike in the US, Black Friday and Cyber Monday fall on normal working days in the UK, so the temptation is there for employees to take time off," said Peter Mooney, head of consultancy at ELAS.

"Workers should not see this as an opportunity to miss work and grab bargains. If employees wish to take time off to do their Christmas shopping, they should use their annual leave to do so.

"In order to mitigate any risk of productivity loss, it is vital that employers have clear internet 'usage at work' procedures and absence policies in place that are communicated with clarity."

"In terms of internet policy, it is a good idea to have engaged content filters on work computers to prevent employees from accessing retail websites."

For more information on the best Black Friday deals, which includes prices for smartphones, tablets, and all other types of products, check out IBTimes UK's live blog.

ELAS' workings for the £117m figure

According to November 2014 ONS statistics, 29.58 million people are employed in the UK, 88% of those bought at least one present online which is 27 million.

The average hourly pay for full and part-time workers is £13.12.

If 27 million take 30 minutes out of their working day to make a purchase, the total cost would be £177.8m.