Shopper numbers across Britain fell 6.7% in the week to 12 June compared with the previous week, which had been boosted by a school holiday and improved weather, researcher Springboard said on Monday.
It said shopper numbers, or footfall, fell 9.0% on high streets week-on-week, by 7.5% in shopping centres and by 0.9% in retail parks.
"UK retail destinations suffered post Bank Holiday blues last week, with footfall dropping back by around half of the uplift recorded in the week before, when the school half-term holiday coincided with the Spring bank holiday and amazing weather," said Diane Wehrle, Springboard's insights director.
Overall UK footfall increased 11.6% in the week to 5 June.
Springboard said week to 12 June footfall was down 18.4% compared with the same week in 2019, before the pandemic started to disrupt traffic.
Non-essential stores reopened in England and Wales on 12 April after more than three months of COVID-19 lockdowns.
They reopened in Scotland on 26 April and Northern Ireland on 30 April. Indoor hospitality was allowed from 17 May.
However, British Prime Minister Boris Johnson is expected to announce on Monday that the end of further restrictions will be delayed by several weeks following concerns about a rapid rise of Delta variant infections.
Springboard said that with the removal of restrictions set to be delayed by as much as a month, footfall is not likely to strengthen significantly over the next four weeks.
"However, it should receive a huge boost in the second half of July, particularly as this coincides with the start of the school summer break," said Wehrle.