High streets and shopping centres up and down the country are expected to be swamped by millions of bargain-hunters as the traditional Boxing Day sales get under way.

Retailers will be slashing prices and opening from as early as 6am in a bid to entice shoppers hoping to pick up a good deal.

But many online retailers tried to stay one step ahead of the competition by offering heavy discounts as early as Christmas Day.

Amazon's UK website said it had seen sales on Christmas Day increase by 263% over the last five years, partly due to the growth in home broadband and the popularity of tablets and smartphones. The retailer is launching further offers later, which include clearance offers and "lightning deals" for a limited time and quantity of stock.

According to MoneySupermarket.com, shoppers in the UK are set to spend a total of £2.9 billion in the Boxing Day sales. A poll for the website found almost four million Britons (8%) plan to head to the high street on Boxing Day in addition to more than five million (10%) who will be searching online.

Shoppers in London could face disruption as Tube drivers prepare to strike over a dispute about bank holiday pay.

Extra buses will be laid on for those travelling to the West End, as well as the Westfield shopping centres in Stratford, east London, and White City, west London, Transport for London said.

Furniture Village said visits to its website on Christmas Day last year peaked at 25,000 at 4pm, with that figure increasing to 50,000 on Boxing Day, suggesting that the majority of customers researched products online before buying from high street stores.

Analysts Experian predict that Christmas 2012 will be the "biggest and busiest ever" for online retailers in the UK, with visits to retail websites expected to reach 126 million, up 31% on 2011. They believe consumers will spend £472.5 million on Boxing Day bargains.

Meanwhile, high-street spending was "acceptable but not exceptional" this festive period, according to the British Retail Consortium. And as more shoppers flocked to the web rather than the shops to buy presents, high-street retailers faced the threat of closure, business recovery group Begbies Traynor warned.