Retail sales growth in Scotland has lagged behind the UK for more than a year.
Total sales for May were up 0.1% on the same month last year but like-for-like sales, which strip out factors such as new store openings, were down 1.2%. The small rise in total sales was much lower than the UK as a whole, which recorded 3.4% growth on May 2011, the 14th consecutive month in which UK sales growth beat that of Scotland.
Ian Shearer, director of the Scottish Retail Consortium, said: "A welcome spell of warmth at the end of May helped lift these figures but the burst of summer didn't bring a boost on the scale enjoyed elsewhere." The "underlying picture remains weak, showing sales falling in real terms for the sector overall".
He said while total sales may have risen last month, it was still "particularly worrying given that they are being compared against poor figures for May in 2011" when sales dropped by 1.1%.
Overall, the new figures show that "consumer caution persisted, household budgets are still under severe strain and most shoppers are committed to buying only what they consider to be essentials".
David McCorquodale, head of retail in Scotland for KPMG, said: "After a dismal April, May was a slightly better month statistically and many retailers will have breathed a small sigh of relief, particularly in the last week of May as the sun shone.
"However, closer scrutiny of the comparisons with last May doesn't make for comfortable analysis. Total sales rose only 0.1% against last May which itself recorded the worst fall we'd seen up to then."
The figures, contained in the latest Scottish Retail Consortium-KPMG retail sales monitor, show total food sales were up 3.8% and like-for-like sales were up 1.9%.
Accounting for inflation, sales volumes are likely to have fallen slightly, the SRC said. Warmer weather at the end of May helped boost sales of summer foods, with shoppers swapping potatoes and stewing meats for salads, ice cream and barbecue foods.
The sunshine also helped clothing sales in the final week of May but this only made up for the poor performance at the beginning of the month. Big ticket household items continued to struggle to sell. Total non-food sales for May were down 3.4% on last year and like-for-like sales fell 4.1%.
Mr Shearer said: "Food retailers had a brighter month with changes in consumer preferences reflecting the shift in temperatures. Demand for roasting joints and soups turned into sales of salad and barbecue meats for the final week.
"Other stores, with supplies of summer fashions, outdoor leisure gear and gardening equipment currently languishing in stockrooms, badly need a more prolonged period of summer weather in the coming months to begin to ease the gloom."
Mr McCorquodale also warned: "A week of sunshine and Scotland's muted celebration of the jubilee will not overcome the underlying issues facing the industry which remains under pressure from a combination of low consumer confidence and squeezed incomes.
"We have seen yet more casualties on the high street in May and more are forecast. The seismic structural changes in the industry would be easier to manage in a better economic environment but retailers do not have that luxury and are therefore hanging on and managing cash and margins as best they can."