Brand owner Premier Foods last month issued a recall on Hearty Oats 800g loaves with a best-before date up to and including June 20 after receiving complaints from five customers in three days.
Hearty Oats production on the affected line was immediately halted pending an investigation.
Refurbishment work on a storage silo exclusively used to store the specific flour and oat blend used in the loaves has now been confirmed as the likely source of the contamination.
The loaves are manufactured at two sites, but only one was affected by the recent recall, said a spokesman.