Smartphone shopping is set to more than treble in value from £13.5 billion to £43 billion over the next four years, accounting for two thirds of all UK online purchases.
But research from Google, PayPal UK and OC&C Strategy Consultants shows that British retailers are failing to keep up with their Asian and American rivals who are adapting more quickly to consumers' shift to mobile.
UK retail sites are around 10% to 25% slower to load than US competitors, though Google research shows that improving load times by one second alone can boost conversion rates by 27%.
PayPal UK mobile commerce director Rob Harper said: "Speed is an important factor in any shopping experience, but when it comes to mobile shopping it's vital.
"Retailers can reduce the time it takes to browse and select a purchase but if it takes too long to pay, they may lose that sale. It's a problem that retailers can easily address."
Only 16% of Britain's top retailers offer customer support through mobile chat versus 41% of the top 100 US sites, and few companies have a presence on messaging platforms despite their growth beyond social networking sites.
In the meantime, around 250 million Chinese consumers made purchases through popular web messaging app WeChat last year.
The report said retailers are experiencing a "fundamental change" in shopper behaviours and demand.
Around 80% of web retail will involve research, transactions and price comparisons through a smartphone by 2020.
About £1.5 billion was spent via smartphones while UK consumers were away from home and work - while travelling commuting and sitting in cafes - while £1.25 billion worth of purchases were made while in a store, £500 million of which was with a competing retailer.
Meanwhile, 39% of consumers trust online information more than an in-store assistant, and a growing number are spending cash in a rush for upcoming events.
Those trends together are expected to drive growth in mobile's share of online retail, the report explained.
"The next evolution of mobile shopping will reduce the consumer journey even further. Contextual commerce will enable consumers to buy things at the point of discovery - whether that's in an email, on a Pinterest page or in a messenger app - rather than needing to click through to an online shop," Mr Harper said.
"Mobile technology is determining the future of e-commerce, and retailers need to act now to prepare themselves accordingly."