Two-thirds of fines imposed on employers of illegal workers have been uncollected in the past five years, the BBC has learnt.
A Freedom of Information request found the Home Office had issued almost £80m in fines but collected £25m.
Restaurants and take-aways incurred the most fines - more than £55m.
Meanwhile, immigration enforcement officers have been taking part in 200 raids and have arrested 101 suspected illegal immigrants across the country.
The Home Office has been running the major illegal workers crackdown.
The data on fines collected by BBC News relates to the civil penalty system through which the Home Office can fine an employer up to £10,000 if they are found to have an illegal immigrant on their books.
The figures show that more than 8,500 penalties totalling £79,300,000 were issued between 2008/09 and 2012/13, but two-thirds of that total remains uncollected.
In the first year that the fines system was in operation, there were £16.7m in fines - but only £1.3m of that was collected. Since then, the amount collected has increased to more than £6m a year.
The businesses that have received the most penalties include restaurants, take-aways, car washes, factories, food producers and small shops. More than 150 supermarkets were also fined for having illegal workers.
In its response to the BBC's FOI request, the Home Office said some of the original fines may have been reduced or cancelled after appeals. Some employers may have also gone out of business - while others could have asked to pay in instalments.
A Home Office spokesman said: "The government is determined to take effective action to reduce illegal migration, and to tackle illegal working. Giving employers hefty fines if they employ illegal immigrants is a key part of this.
"As part of our upcoming Immigration Bill, we will take even tougher action against businesses which use illegal labour and will double the fines levied against rogue employers. The Bill will also strengthen our ability to enforce unpaid fines."
In the last financial year, there were more than 5,000 illegal workers operations resulting in some 4,500 arrests.
Speaking about Thursday's enforcement operations, immigration minister Mark Harper said: "Today's arrests highlight the routine work we are carrying out every day to stamp out illegal working.
"We are sending a clear message to employers who choose to use illegal labour: we will find you and you will pay a heavy penalty.
"Illegal working undercuts legitimate businesses and is often linked to exploitative behaviour such as tax evasion and harmful working conditions. We will not allow the growth of a shadow economy for illegal migrants."
- FOI data conducted by BBC Researcher Laura Francis.