LONDON—British retail sales unexpectedly fell for a fifth month in a row last month, despite a rush of motorists filling their cars with fuel, adding to signs that Britain’s economic recovery is losing momentum. Sales volumes dropped by 0.2 percent in September, official figures showed on Friday, as supply-chain problems led to gaps on store shelves. It marked the longest run of consecutive monthly falls since the series began in 1996 and bucked economists’ expectations in a Reuters poll for a rise of 0.5 percent. This softness adds to the conundrum faced by the Bank of England over whether to counter rapidly rising inflation by raising interest rates next month, or instead to wait for the economy to recover more of the ground it lost to the pandemic. “Overall, the data support our view that the economic recovery stalled in September,” said Bethany Beckett, UK economist at Capital Economics. Retail …