Amazon UK has seen more than 1mln daily mobile app users desert the platform since the start of the year.
Data compiled by mobile analytics firm GWS just ahead of Amazon's Prime Day said that daily shoppers declined from 9.3mln in January to 8.3mln.
GWS said the fall was put down to the rapid emergence of other e-commerce rivals as opposed to customers scaling back on spending amid a cost-of-living crisis.
Temu, which launched in the UK in April this year, has seen daily mobile users rise to around 3.5mln in one month, although growth has plateaued since May.
Shein, the Chinese fast fashion retailer, has also doubled its daily users this year to 2mln, the same amount of daily visitors as second-hand clothing site Vinted.
“For years Amazon has gone all but unchallenged when it comes to offering the complete e-commerce package to consumers, and much of its success has been via its mobile app,” said GWS CEO Paul Carter, quoted on the website Retail Gazette.
“However, mobile analytics data from GWS shows how brand loyalty on retail shopping apps moves quickly during challenging economic times, and retailers need to stay on top of the curve or risk getting left behind”.