Pieces of glass have been reported in packs of Kingsmill sliced bread, the Food Standards Agency said.
Allied Bakeries is looking into suspected cases of malicious tampering at its plant in Orpington, Kent. It is the third time in just over a year that glass has been reported in packs of Kingsmill.
The Food Standards Agency is urging customers to be vigilant for possible tampering.
An investigation was launched after the first cases of needles and glass in bread packs were reported in September last year. That was followed by more reports of tampering around two months ago.
Allied Bakeries confirmed glass had been found in one further pack of Kingsmill. The firm is still investigating customer reports of glass in three other packs.
The Food Standards Agency said the glass was found inside the packaging and not in the bread itself.
It is still working with police, the London Borough of Bromley and Allied Bakeries to find out who is behind it.
The Orpington bakery distributes to retailers across the south east of England.
An Allied Bakeries spokeswoman said: "Customers notified us of the contamination and we are trying to find out whether or not it happened in the factory itself or further along the line."
Customers who find any foreign objects in Allied Bakeries bread should immediately contact the careline on 0800 1970110.
