Mobile devices accounted for 51% of online sales between November and January, a substantial increase on the 45% recorded in the previous quarter and up from 40% on the same period the year before, according to the latest figures from the IMRG Capgemini Quarterly Benchmarking report.
Tablets accounted for 33% of sales, smartphones for 18% and desktop or laptop computers for 49%.
Visits to retail websites via mobile devices accounted for two-thirds (66%) of traffic over the last quarter, up from 63% in the previous quarter and 53% on the same time last year.
The report said the leap appeared to be driven by increased confidence in using smartphones for online shopping, likely helped by the trend for larger screens but also by significant improvements made by many mobile retail sites.
IMRG chief information officer Tina Spooner said: "Smartphones have played an important role in the overall online shopping process for a long time - often used for research and comparison on the go - but over the past year they have really started to become a major component of the checkout process too and that is what is driving this leap in mobile penetration.
"There is also the fact that we increasingly use our smartphones for managing so much of our lives. It's only logical that completing purchases on retail sites would gravitate over to these devices as well."
Richard Tremellen, retail insight and data specialist at Capgemini, said: "This is a very significant milestone in the history of the online retail sector and reflects the work retailers have put into improving the customer experience on smartphones."
John Lewis online trade director Mark Felix said: "Mobile is undoubtedly our fastest growing sales channel.
"Importantly though, mobile is becoming the glue between our shops and online. Increasingly, customers are using the two channels combined and for example use their mobiles to check ratings and reviews and further product information when in-store."
Andy Harding, chief customer officer at House of Fraser, said: "We are not surprised by the growth of mobile devices reported today.
"Mobile has been clearly identified as a priority area for House of Fraser, with over 40% of online sales coming from a mobile device last year and growing at a rate of over 100% year-on-year."