"Customers are still worried about jobs and their own finances, so they are keeping spending under tight control," said Stephen Robertson, director general of the consortium, which compiled the figures along with consultancy firm KPMG.
But there were some sectors that saw sales increase.
Non-food internet, phone and mail-order sales, for example, jumped by 10.8%.
Food, clothing and footwear sales also rose slightly.
Total sales in March - which includes new store space - were up 0.6% from a year go.
"A slight air of pre-spring optimism tempted customers to buy new season clothing and women's footwear," said Mr Robertson.
"But this is unlikely to be the basis of any sustained improvement," he cautioned.
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