B&M blamed disappointing weather in May, which "held back" sales of outdoor seasonal products, for the decline.
The company flexes its range according to the selling season, so gardening is stocked in spring.
More than half B&M stores are in the north of England and Scotland, where weather was worse than in the south this spring.
Group sales revenue for the 13 weeks ending 27 June 2015 increased by 25.3 per cent (2014: 31.9 per cent) on a constant currency basis. On an actual currency basis, total sales revenue increased by 24.4 per cent (2014: 31.9 per cent) to £456.6m (2014: £367.0m). The UK business' sales revenue for the 13 week period increased by 22.8 per cent to £418.8m (2014: £341.2m).
At the end of the first quarter of 2015, the group's UK business had 450 stores trading, having opened 25 net new stores over the last 13 weeks, against the context of its UK store target of 850 stores. B&M expects to open 80 net new stores this year.
Chief executive Simon Arora said: "As consumer spending improves, it is apparent that shoppers remain keen to spend with value retailers and we continue to win market share."