The three-month rolling average complied by the organisation and KPMG showed a steeper like-for-like decline, of 2.4 per cent to the end of June, compared with a 1.5 per cent drop in sales for the three months to the end of May.
Total sales were up 3.5 per cent in the month, with a spate of good weather boosting clothing and footwear sales.
The figures were released a week earlier than usual by the BRC in an attempt to add momentum to the business campaign for lower interest rates.
BRC director-general Kevin Hawkins (pictured) said: 'Retailers cannot rely on good weather to bring in customers. There is an urgent need for an early cut in interest rates to prevent a continued decline.'
KPMG head of retail Helen Dickinson added: 'Looking ahead to the second half of the year, the comparatives are weaker and I'm sure that many retailers are hoping this will bring some respite.'