Online clothing sales increased 4.5% year on year last month, but retailers failed to match June’s increase of 15.7%.
The latest IMRG Capgemini eRetail Sales Index show July also fell short of the 8.6% five-year average.
This is despite the hot weather and online shopping events, such as Amazon Prime Day on 15 July.
Online sales of menswear were up 10.4% year on year, while womenswear and accessories decreased by 8.7% and 13.3% respectively.
Total online sales were up 4.4% year on year in July which, was the “lowest ever” July increase.
Total sales were down 5.7% compared with June.
IMRG strategy and insight director, Andy Mulcahy, said: “Online sales growth had been subdued throughout the first half of 2019, but in June there seemed to be a bit of a bounce-back.
“However, there is now evidence that the June performance was artificially inflated by heavy discounting. The clearest indication came from clothing, which recorded its strongest growth in over a year in June, but the average basket value was down by around 25%, suggesting that the rise was driven by discounting.
Bhavesh Unadkat, principal consultant at Capgemini, added: “July disappoints after a better June performance.
“The outlook remains uncertain for retailers in the second half, as consumer spending is cautious and confidence low.”