The number of people choosing to buy groceries online fell by 81,000 in the period compared with the same four weeks last year, with the value of digital baskets falling 8% to an average of 80 pounds ($109.4) per shop, it said.
The channel accounted for 13.3% of the total UK grocery market, down from 13.4% in Kantar's June report.
The researcher said overall grocery sales fell 5.1% year-on-year over the 12 weeks to July 11, but rose 10.9% compared with the same period in 2019 - before the COVID-19 pandemic started to disrupt trading.
It said shoppers spent 3 billion pounds more on groceries than they did during the same period in 2019.
Britons spent 1.2 billion pounds on alcohol in the four weeks to July 11 as they gathered together to watch the Euro 2020 soccer tournament - a 24% rise on the same period in 2019.
All of Britain's big four grocers - market leader Tesco, Sainsbury's, Asda and Morrisons, recorded sales decline versus last year.
Grocery prices fell 1.5% over the 12-week period, Kantar said, with prices rising fastest in markets such as savoury snacks and cat food while falling in fresh bacon and vegetables.
($1 = 0.7314 pounds)
(Reporting by James Davey; editing by Richard Pullin)