A number of arrests were made after Home Office raids at two Chinese restaurants.
Acting on intelligence, immigration enforcement officers from the Home Office visited two businesses in Desborough on Wednesday night.
Staff were questioned to check if they had the right to live and work in the UK.
Officers made one arrest during the first raid at the Great Wall Chinese Restaurant in Station Road.
They arrested a 47-year-old Chinese man who was an illegal entrant to the UK.
He was released on immigration bail while action is taken to remove him from the country.
The second raid was at Dragon City Chinese Restaurant in High Street, where officers arrested three Chinese men.
They included a 29-year-old who had overstayed his visa and two 26-year-olds who had entered the UK without leave.
A 27-year-old Chinese woman who had overstayed her visa was also arrested.
All four were released on immigration bail while action is taken to remove them from the UK.
Alison Spowage, assistant director of East Midlands Immigration Enforcement, said: “This operation is an example of the regular visits that Home Office immigration enforcement carries out across the East Midlands to businesses believed to be employing illegal workers.
“Employers must, by law, check whether the people they hire have the right to work in this country.
“Businesses failing to carry out the correct checks can expect to face heavy financial penalties.
“Illegal working defrauds the taxpayer, and cheats legitimate job seekers out of employment opportunities.
“I would urge members of the public with information about suspected illegal working practices to get in touch.”
Both businesses were notified that they could be liable for a civil penalty of up to £20,000 per illegal worker arrested if proof is not provided that the correct right-to-work checks were carried out.
This is a potential of up to £20,000 for Great Wall and £80,000 for Dragon City.
Information to help employers carry out checks to prevent illegal working can be found at: https://www.gov.uk/government/collections/employers-illegal-working-penalties.