Fifteen illegal immigrants were found working at a Morrisons store during a check by Border and Immigration Agency officials.
Inspectors found they had used forged documentation to get through Morrisons' vetting procedures at the St Albans store.
Morrisons said it would avoid prosecution as the immigration officials had accepted the forgeries were extremely difficult to detect.
"The standard of the documentation was such that the agency agreed it would have been impossible for us to identify it as fraudulent."
The 15 workers were currently being processed through official channels, said the Home Office.