Total like-for-like retail sales fell back 1.5% on last year. This was the second consecutive month of like for like declines, which has not occurred for three years.
An early Easter and snow versus last year's heat wave makes straight comparisons difficult but the BRC said that underlying trading had remained tough.
Fashion sales were worse than March despite heavy discounting and promotions - the seventh consecutive monthly decline. Kidswear outperformed menswear and womenswear and value ranges held up better than sales of spring ranges. There were still some gains in the designer sector.
Footwear sales also fell further, to the worst level for eight years. Kid's footwear, which had held up until March, showed its largest decline for three years. Ankle boots and smart work shoes performed better than spring sandals because of weather. Premium footwear did show some gains.
Department stores and the mail order sector were also hit by the slow fashion and footwear sales and bad weather.
However the data period ended on May 3, the Saturday of the Bank Holiday weekend, when warm weather is believed to have started to lift some fashion sales. The full impact of Bank Holiday weekend will be part of the May BRC sales monitor survey.
Stephen Robertson, director general, British Retail Consortium, said: "This is the first time in three years we've had two months in a row of year-on-year falls in like-for-like sales, further evidence that hard-pressed customers are really watching the pounds. With higher fuel and utility bills eating away at people's spare cash, they are concentrating on essentials like food. Despite heavy discounting, clothing and footwear were at their weakest for at least eight years..."
Robertson added: "Retailers will look to the recent sunny weather to provide some boost but with the economic fundamentals remaining weak, there seems no reason for these tough trading conditions to improve soon. However, this is a good time for the savvy shopper to pick up some bargains."