Raids by immigration officers on car washes in Hounslow, west London, found 20 people who were suspected of working illegally.
Three suspected illegal immigrants from Albania were arrested and 17 more Eastern Europeans could face deportation for allegedly breaching European Union working regulations at the premises in Isleworth and Cranford.
Immigration enforcement officers swooped on ABW Car Wash, in London Road, Isleworth, on Friday April 15.
A Home Office spokesman said three male workers from Albania were found to have entered the UK illegally and were detained pending their removal from the country.
He said three Romanian men and one man from Bulgaria were found to be breaching their EU treaty rights by working cash in hand and not paying taxes or National Insurance.
They were served with enforcement papers, giving them the right to appeal or 30 days in which to demonstrate they are now meeting EU working conditions. If they fail to do either they could be removed from the UK.
ABW Car Wash was also served with a notice warning the firm it could be fined up to £20,000 per illegal worker found unless it could prove the necessary right-to-work checks had been carried out.
The previous Friday immigration enforcement officers had visited Julian Car Wash, in Bath Road, Cranford.
A Home Office spokesman said they found 11 men from Romania and two from Bulgaria working there cash in hand, meaning they were not paying tax or National Insurance, in breach of EU treaty conditions.
They were also given the right to appeal or 30 days in which to show they are now exercising their treaty rights.