Supermarkets and high streets across Britain will be swamped as shoppers make their final purchases before Christmas.
Today is expected to be the most popular day for festive grocery shopping, while an hour at lunchtime tomorrow is believed to be the peak time for high street shopping.
Supermarket aisles will be filled today as some 36% of consumers plan to do their main Christmas food and drink shop, according to a survey by BBC Good Food released last week.
This was followed by 35% who are due to stock up on festive treats tomorrow.
BBC Good Food editor Gillian Carter said: "Just as Black Friday and Manic Monday saw record numbers buy gifts, we have identified Mayhem Monday December 22 as the peak day for food shopping.
''Our food behaviour radically changes in the run up to Christmas. We are more likely to visit independent shops, less likely to shop online and more likely to plan ahead.''
Meanwhile Visa Europe expects some £1.3 billion to be spent using its cards tomorrow, making it the busiest day on the UK high street in the run up to Christmas.
Some £916,667 will be spent every minute or £15,278 every second on Visa cards, the company predicts.
Kevin Jenkins, managing director at Visa Europe, said: "Black Friday kick-started Christmas on the high street and online this year but the busiest bricks and mortar day will likely remain in its traditional slot close to Christmas.
"Retailers' multi-channel approach should cause a surge in footfall from click-and-collect sales too, with the opportunity for further shopping in-store when consumers arrive.
"Across Tuesday December 23 we are likely to see £1.3 billion spent in total. Lunch hour should prove the most popular time for a shopping trip, either for last minute gifts or final ingredients for Christmas dinner."
Thirty-four million transactions are expected to take place which is 7% higher than in 2013, while spending is predicted to rise 6%.
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