Belgian customs officers arrested 45 people on Wednesday as part of the country's biggest operation into cigarette counterfeiting.
The country has become a hub for cigarette trafficking to neighbouring countries.
Officers arrested 17 people of Ukrainian and Bulgarian nationality during a pre-dawn raid on a sweatshop in an industrial area of Aartselaar, near the northern city of Antwerp.
The men were housed in a dilapidated dormitory in the warehouse, working in shifts to dry tobacco and run it through rolling machines.
"This warehouse operates 24 hours a day, and the workers don't leave it, so as not to give any sign of suspicious activity to the neighbourhood," Florence Angelici, spokeswoman for the Belgian finance ministry, explained.
"And so they will sleep, work, eat, shower on site, without going out for weeks, even months," she added.
A large stock of cigarettes was ready to leave for the UK, in imitation branded packs with the health warnings that are common in the country.
28 million cigarettes
A total of four illegal production sites were discovered, as well as several sites used for logistics, grinding tobacco or storing cigarette paper, filters and glue.
In addition to Aartselaar, workshops were also located in Tongeren, Eeklo and Frasnes-lez-Anvaing —respectively in eastern, northern and western Belgium and providing easy access to the British, French and Dutch markets.
A total of 45 people were arrested, some of whom were already known for similar offences in other European states, the finance ministry said, adding that "most" were Ukrainian nationals.
According to a first estimate, more than 28 million cigarettes were seized, but the inventory is still underway, the ministry added. The contraband cigarettes were sold under the Marlboro, Richmond, Prince and Regina brands.
The number of counterfeit cigarette production sites closed by the Belgian authorities now stands at seven since the beginning of the year, compared to five for the whole of 2020.
Counterfeit cigarettes 'bring a lot of money'
More than 400 million illegal cigarettes were seized by Belgian customs last year, compared to 110 million in 2018.
Belgian customs administrator general Kristian Vanderwaeren described the operation on Wednesday as the biggest ever conducted by his administration.
"The black market (of cigarettes) brings in a lot of money, as the taxes are very high," he explained. Here "a packet of cigarettes costs about €8, and out of that, six to seven euros are excise and VAT".
Packs of contraband cigarettes sell for €4-5.
"Half-price cigarettes are not legal cigarettes, and the product, whether it is tobacco or filter, does not comply with health standards," Vanderwaeren warned.
Moreover, the sales "feed criminal organisations, which then use this money to organise trafficking in women and drugs", the official flagged.