A butchers is facing a potential fine of £30,000 after being raided by the UK Border Agency.
Officers visited Islamabad Halal butchers on High Street, Thornton Heath, early on June 27, and carried out checks on staff to ensure they had the right to live and work in the UK.
Three workers were arrested, two Pakistani men aged 22 and 44, and a 32-year-old Indian man.
Enquiries found the 22 and 32-year-old men had both overstayed their visas, while the 44-year-old is believed to have entered the country by deception.
The business was served with a notice warning it could be fined up to £10,000 for each illegal worker arrested, unless it can prove to the UKBA they correct right-to-work checks were carried out.
Paula Tucker, from the UK Border Agency's Croydon Local Immigration Team, said: "We carry out hundreds of operations like this every year across London and where we find people who are in the UK illegally we will seek to remove them.
"Illegal working has a serious impact on communities, undermining legitimate businesses and taking jobs from those who are genuinely allowed to work.
"We are happy to work with businesses to let them know what checks need to be done on staff, but those who do break the law should know we are out there looking for them and they will face heavy fines."
Anyone who suspects that illegal workers are being employed at a business can contact Crimestoppers on 0800 555 111 anonymously or visit crimestoppers-uk.org.