Allied Bakeries has almost ruled out sabotage on its production line after glass and needles were found in Kingsmill loaves.
Three contaminated loaves have been reported in the last fortnight, although the objects were discovered inside the packaging rather than the bread itself.
Allied has DNA-tested the 300 staff at its Orpington plant, where a police investigation is focused. Security has been stepped up, with staff restricted to certain areas and CCTV cameras installed.
Said a spokeswoman: "We’re still not sure where contamination is taking place but we think it’s not on the production line."
Consumers found fragments of glass, sewing needles, chewing gum and plastic in packs of sliced bread produced at the Orpington site in July 2005, and then again in September that year. This prompted a police investigation into the contamination.
Allied Bakeries is working closely with the Food Standards Agency and the police on the latest investigation. The spokeswoman added: "We take consumer safety very seriously and continue to take all necessary action to ensure our products are safe."
She urged consumers to be cautious and report any problems to the company careline.