For the five weeks to 29 June, Scottish sales decreased by 3.1% on a like-for-like basis compared with June 2018.
Non-food sales saw the worst decrease but – after previously remaining robust – Scottish food sales were also in trouble.
Paul Martin, UK head of retail at KPMG said: “Food retailers are far from immune from the difficult trading conditions, with another flat performance.
“The rate of decline across the sector is now outpacing the rest of the UK. With signs that trend could be set to continue, the coming months could be particularly difficult for the industry.”
Ewan MacDonald Russell, head of policy and external affairs at the SRC said: “Another difficult month for retailers as sales struggle to match the heights of last summer.
“A real-terms fall of 2.2% is very troubling for the industry at a time of year when consumers should be feeling more optimistic.
“There is a clear trend in recent months of a slowdown in consumer spending. The consequence of the unending Brexit gridlock and stagnation is reduced confidence amongst retailers and consumers.”