The first needle was discovered last week but it wasn't until today, after three more were reported, that thousands of Asda and Asda Farmstores loaves were withdrawn.
Greater Manchester Police were called in to investigate how the needles got into loaves after the first customer complained to Asda last week. When a further three complained today, the company pulled thousands of loaves from 56 stores in Yorkshire, Greater Manchester, Lancashire, Cheshire, Merseyside and North Wales.
Police are now investigating at Allied Bakeries in Stockport, which supplies Asda shops in the North West.
A spokeswoman for Asda said: "The needles were put in after baking and, although there is no definite proof yet that it was at the bakery, it seems likely, and a police investigation has begun.
"All we are concerned about is the welfare of our customers so we have made sure all loaves made by Allied Bakeries are removed from the shelves."
Pat Lally, the operations director for Allied Bakeries, said:"We take this very seriously and have mounted a further investigation. We have not yet been able to identify where the contamination occurred, however we have made a number of steps to tighten our already rigorous safety procedures at our bakeries.