Five Chinese nationals have been caught working illegally after a raid on a Thai restaurant in Shropshire, the UK Border Agency said.
Officers visited the Siam Cottage, Tontine Hill, Ironbridge, on Wednesday.
Of the six staff on site, only one was entitled to work, the agency said.
Siam Cottage was warned it may be fined up to £50,000, unless bosses can prove they carried out the correct right-to-work checks on their employees.
The agency said one worker was a known immigration offender who was arrested and detained while it takes steps to remove him from the UK.
The remaining four illegal workers were granted immigration bail and will have to regularly report to a UK Border Agency office or police station.