THE UK Border Agency is holding a 47-year-old illegal immigrant found working at the Blue Sea Food Company in Paignton.
The man, from the Philippines, is in detention awaiting removal from the country after he was arrested in a raid.
Immigration checks revealed he had stayed in the UK illegally after his visa expired.
The arrest shocked management at the Blue Sea Food Company.
Dawn Spencer, operations director, said: "It was a surprise to us because we thought we had been doing all the right checks.
"The worker had overstayed his visa and the company checks didn't pick it up. We have now changed our system.
"We have spoken to the Border Agency and they are sending somebody down to give us a bit of extra training.
"We employ 125 staff and are a growing company. We employ lots of people from the EU and other foreign nationals and my advice to other employers in the Bay if they are as keen as we are to cooperate with officials is to get on to the Border Agency website and download advice to employers.
"The worker in question was in his second season with us and it was a real shock to see him here one day and gone the next.
"He was a well-liked employee who worked as an operative in the factory and got on well with the others.
"The following day we got together everybody's passports and visas like we should have had to check the details. In short, we've learned our lesson."
Further UK Border Agency visits were carried out on June 28 on two more businesses in Paignton and Newton Abbot but no illegal workers were found and no arrests made.
Richard Taylor, of the UK Border Agency, said: "We want the public to get in touch with us if they have any information about suspected illegal workers or the businesses employing them.
"We are sending a simple, clear message to people who are in the country illegally — more raids are planned and you will be caught."
Anyone who has information on suspected immigration offenders can log on to www.ukba.homeoffice.gov.uk/aboutus/contact/report-crime