The fall comes one month after BDO's High Street Sales Tracker (HSST) registered the largest drop in sales since the height of the recession.
Apart from a boost during the January sales season, the HSST has shown negative growth since October 2015.
Lifestyle sales were particularly poor, falling 2.4 per cent compared to May 2015. Fashion sales fell by 1.9 per cent after a very difficult period for high street clothing stores; it emerged last week that BHS is being "wound down", after Austin Reed announced it is shutting up all its shops.
Buyers preferred to buy online in May; non-store sales rose 18.5 per cent. This follows analysts' concerns that footfall on the high street may never bounce back.
Sophie Michael, head of retail and wholesale at BDO, said: "A fall in retail sales is often sparked by consumers choosing to spend their disposable income on things like eating out, but the evidence is that people are reducing spending across the board.
"Things have been tough for retailers since the end of last summer and there's not much to suggest confidence will pick up soon. Retailers will have to be strategic with their discounting and confident their product is strong enough to appeal to their customers when they decide to spend."