Online sales rose by 14 percent over the last three months to hit £27.3bn, aided by an increase in online shopping on smartphones and tablets.
The sales spike was driven by an 18 percent increase in online transactions in June compared with the same month a year earlier, which equated to £9.3bn in sales.
The Index also attributed the rise in part to a 2.7 percent hike in online sales in May, with sales of £9.1bn, helped by people shopping for warm weather clothing in the run up to the summer months.
The travel sector further contributed to the rising sales performance, seeing 25 percent growth in online transactions, the highest the sector has enjoyed so far this year.
Purchases made on smartphones and tables were also in rude health. The Index showed a 57 percent increase in sales made on mobile devices in June. Smartphones saw double the amount of purchases compared with tablets.
The figures revealed a growing gap between online-only retailers, and multichannel retailers with a physical and web-based presence.
Online-only retailers saw 13 percent year-on-year growth in June, while multichannel retailers saw 21 percent.
Steve Hewett, head of retail customer engagement and loyalty at Capgemini, explained that this divide in retailers is down to how they offer services to customers.
"I'm particularly interested to see the impact that click-and-collect services have had on the performance of multichannel retailers compared with their online-only counterparts," he said.
"Capturing their customers' delivery needs has helped build a stronger connection between retailer and consumer, which is ultimately being reflected at the cash register.
"It will be key for the online community to capitalise on the current consumer confidence and find a way to build the same level of connection."
Hewett was also positive about the outlook for online retail for the second half of 2015. "The significant growth in the market will be very reassuring to UK retailers who have thus far experienced a fairly turbulent 2015," he said.
"With a relatively settled post-election economy, consumer confidence should remain high and we can look forward to a solid [second half of the year]."
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