During the same period, footfall fell to its lowest January rate in five years, according to new figures.
The fall in annual growth is one of multiple indicators of a maturing online retail market, says study.
Online shopping on Boxing Day is forecast to exceed a billion for the first time.
New data also shows that the percentage of orders being delivered on time was down on last year.
Latest figures from IMRG and Capgemini indicate a pre-Black Friday lull.
The retail sector will hope that the discounting period will build momentum ahead of Christmas.
Consumers are becoming more price sensitive, comparing deals online and sticking to essential purchases.
The rise of click and collect also drove double digit multichannel retail growth for the first time in two years.
Historical analysis indicates a strong correlation between elections and a dip in consumer spending.
Warning signs that sustaining this rate of online growth throughout the whole year may be a challenge.