Five people have been arrested on suspicion of working in breach of visa conditions after a raid by UK Border Agency staff in Milton Keynes.
Following a tip-off, officers entered the premises of a party supplies firm on Clarendon Drive in Wymbush.
Four men - two Indians, a Sri Lankan and a Pakistani - were arrested on suspicion of immigration offences.
A 25-year-old Ghanaian man was arrested on suspicion of obtaining a counterfeit Belgian passport.
The Ghanaian was held by police, while the rest remain in immigration custody.
The employer Partyrama now has 28 days to prove to the UK Border Agency that the correct right-to-work checks were carried out, or it could face fines of up to £10,000 per illegal worker, an agency spokesman said.
"We're committed to removing people who have no right to be or work in the UK, and we're working closely with other agencies and local authorities to do that.
"The vast majority of companies are law abiding, and we're happy to work with businesses to ensure the right checks are done on staff.
"But those who do break the law will face action," he said.
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