B&Q sales fell 6.7% during the three months to 16 July as a result of the demise of the Focus DIY chain, according to a trading update by parent company Kingfisher.
In a trading update, Kingfisher said that the demise of the Focus DIY chain, and its rush to cut prices in order to clear stock had caused the fall in like-for-like UK sales.
Ian Cheshire, chief executive, said: "These are testing times for retailers," and warned more of "challenging" trading conditions ahead.
Like-for-like sales at B&Q in the 11 weeks to 16 July fell 6.7%. Overall, Kingfisher's UK sales dropped by 5.5% with a good performance by trade supplier Screwfix offsetting some of the downturn at B&Q.
Sales at Kingfisher's two French chains, Castorama and Brico Depot were much stronger, adding 3.7% on a same-store basis during the 11 weeks.
Despite the difficult trading conditions in the UK and Republic of Ireland, Cheshire said that it provided "an opportunity for strong businesses such as ours to strengthen their position".
"We expect to emerge from this year in excellent shape and well prepared to start delivering the next phase of our growth plans," he said.