Like-for-like sales at B&Q, which has 330 stores, fell 6.7 per cent in the 11 weeks to 16 July.
Kingfisher’s UK sales overall dropped by 5.5 per cent, with a good performance by trade supplier Screwfix offsetting some of the poor performance at B&Q.
The company claimed B&Q was hurt by rival Focus DIY going into administration and cutting prices to clear stocks.
Kingfisher chief executive Ian Cheshire said: “These are testing times for retailers. The UK market remains challenging compounded by disruption in the second quarter from heavy stock clearance activity by a major competitor closing down.”
Sales at Kingfisher’s two French chains Castorama and Brico Depot were much stronger, adding 3.7 per cent on a same-store basis during the 11 weeks.
Across the Kingfisher Group, including its outlets in eastern Europe, sales were 0.5 per cent down.
Cheshire added: “We expect to emerge from this year in excellent shape and well prepared to start delivering the next phase of our growth plans.”
Shares in Kingfisher jumped 5.6 per cent yesterday.