Figures from the DIY chain’s parent company, Kingfisher, show that like-for-like sales in the three months to April 30 dropped 9%.
Total sales at B&Q in the period also fell, by 8.8% to £828 million.
The freezing weather hit footfall and sales in weather-related categories, while some stores were forced to shut.
Kingfisher boss Veronique Laury also pointed to weak consumer demand as denting the firm’s performance.
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“It was a challenging start to the year, with exceptionally harsh weather across Europe and weak UK consumer demand.
“This impacted footfall, especially sales of weather-related categories. February and March were particularly affected, with sales improving over the course of April and into May.
“Market conditions continue to be mixed. The UK is uncertain.”
Sister firm Screwfix, the group’s top performer, booked a 3.6% rise comparable sales.
But it was not enough to stop the group from posting a 4% slide in like-for-like sales in the quarter.
Total sales at the group were down 1.2% to £2.8 billion, while UK sales dipped 3.7% to £1.2bn.
B&Q is in the middle of an overhaul, which has seen it shut 65 shops and slash around 3,000 jobs in the UK and Ireland over the last two years.
But Ms Laury insisted her turnaround was beginning to bear fruit.
“We are on track to deliver our One Kingfisher strategic milestones for the third year in a row and we continue to see tangible delivery of our plan,” she said.
“Around 40% of our ranges are now unified and continue to be well received by customers.
“Sales of these ranges, excluding outdoor products, are up, and we expect to grow the full year Group gross margin, after clearance costs.”