U.K. consumers will spend 4.6 billion pounds ($7.3 billion) online over the two weeks beginning Dec. 3 and Dec. 10—20% of that total, 920 million pounds ($1.5 billion), will stem from mobile commerce, according to the IMRG Capgemini eRetail Sales Index.

In the first quarter of 2012, consumers made 8.2% their online purchases through a mobile device; by the end of the fourth quarter, that figure will reach around 20%, say consulting firm Capgemini and trade association Interactive Media In Retail Group, or IMRG.

“What we are seeing in this 20% mobile forecast is a shift in the way that consumers interact with brands online,” says Tina Spooner, chief information officer at IMRG. “While it is still common for consumers to browse the shops at weekends before making the final purchase online at Monday lunchtime, we are increasingly seeing the popularity of ‘second-screening,’ where people browse on their mobile devices in front of the TV.”

This year mobile commerce has become a must-have for retailers, says Chris Webster, head of retail and technology at Capgemini.

IMRG and Capgemini forecast that the peak holiday shopping weeks this year will begin on Monday, Dec. 3, and Dec. 10, a week later than in 2011. The reason for this is that payday for many people will fall after the final Monday in November, which is Nov. 26, the firms say.

The two Mondays falling within the peak period are expected to be the biggest holiday shopping day for many online retailers, but the IMRG Capgemini e-Retail Sales Index has recorded heightened sales activity over the whole two-week period during the past few years. And retail executives are not unanimous on what they think will be the biggest day this year. In October IMRG surveyed 25 senior e-commerce professionals to see when they thought their peaks would be: 44% said Dec. 10, 22% said Dec. 3, 22% said Nov. 26, and 12% cited other days.