Numbers of shoppers in Britain's high streets, retail parks and shopping centres in May were 81.6% lower that the same month a year ago as most non-food stores remained closed due to the coronavirus lockdown, the British Retail Consortium (BRC) said.
The footfall data showed a slight improvement on the 84.7% decline seen in April as some garden centres and homeware shops reopened during May.
Other non-essential stores can open their doors on Monday as long as they are able to comply with social distancing rules.
Prime Minister Boris Johnson, keen to see the UK economy start recovering from a coronavirus hammering, said on Sunday he was "very optimistic" about stores reopening.
"I think people should shop and shop with confidence but they should of course observe the rules on social distancing and do it safely as well," he said after touring the Westfield Stratford City shopping mall in east London.
BRC chief executive Helen Dickinson cautioned that the reopening of non-essential shops was unlikely to bring immediate relief for retailers who had been under immense pressure for three months.
"A mix of low consumer confidence and limits on the number of people able to enter stores mean that many shops will continue to suffer lower footfall – and lower sales – for some time to come," she said.