That makes it the biggest fall in that time period since January 1995, sparking concerns about the future of high street heavyweights in a tough trading environment.
Food sales dropped for the first time since June 2016, and there were also falls in sales of clothing, footwear and outdoor goods compared with a month earlier.
And there was an even bigger fall of 3% last month compared with the same period a year ago in comparable store sales – the steepest drop in a year since December 2008.
Helen Dickinson, chief executive of the British Retail Consortium (BRC), warned the poor figures could lead to further store closures and job losses in a year that has already seen several retailers go bust – including HMV, Debenhams and LK Bennett.
Yet in the three months to May, food sales increased 0.8% on a like-for-like basis and 1.9% on a total basis, thanks to stronger trading in March and April.
During the same three-month period, in-store sales of non-food products fell 2.7% on both a total and like-for-like basis – worse than the 12 month total average fall of 2.4%.
Paul Martin, UK head of retail at KPMG, pointed out that growth in online sales was also slower than usual.
He warned: “The extremely low growth online is real cause for concern, especially with almost a third of all non-food sales today being made online.
“This trend has continued to manifest itself over the last year and requires real focus from the retail community.”