Consumers in the US have spent more than $21.95 billion on holiday purchases bought from retail e-commerce sites during the first 40 days of the November to December holiday season to date - a 12 percent increase from last year.

"With more than $5 billion in spending this past week, it's clear that while deal-seeking shoppers may have driven stronger than anticipated spending early in the season, Americans continue to demonstrate a significantly greater willingness to spend online this year than in seasons past," said comScore chairman Gian Fulgoni.

Online shopping in the US is expected to peak in the days leading up to Christmas, says comScore, with "Green Monday" tipped to be one of the biggest spending days on the calendar.

Green Monday was a term first coined by online retailer eBay and is used to describe the day that tended to be the heaviest online spending day of the year, a Monday typically occurring in the second week of December.

"This coming week, beginning with Green Monday, should see some of the heaviest online shopping activity of the season and we expect at least one more day to surpass the billion dollar spending threshold," said Fulgoni.

According to comScore, November 29, 2010 (the first Monday after Thanksgiving in the US, referred to as "Cyber Monday") is, so far, the only billion-dollar online spending day on record.

A second comScore study released in late November found consumers in the UK to be the most prolific online shoppers in Europe. France, Germany and Ireland followed closely behind with around 80 percent of shoppers in those countries visiting online stores in October.

The Ten Heaviest US Online Retail Spending Days on Record:

  1. Monday, Nov. 29, 2010 (Cyber Monday) - $1,028,000,000
  2. Monday, Dec. 6, 2010 - $943,000,000
  3. Tuesday, Dec. 15, 2009 - $913,000,000
  4. Tuesday, Nov. 30, 2010 - $911,000,000
  5. Wednesday, Dec. 8, 2010 - $901,000,000
  6. Thursday, Dec. 9, 2010 - $898,000,000
  7. Wednesday, Dec. 9, 2008 - $887,000,000
  8. Monday, Nov. 30, 2009 (Cyber Monday) - $887,000,000
  9. Tuesday, Dec. 1, 2009 - $886,000,000
  10. Monday, Dec. 10, 2007 (Green Monday) - $881,000,000