Social affairs ministry inspectors found problems at one in five mushroom farms inspected between 2011 and 2014, despite extra efforts to stamp out exploitation.

The ministry set up a special team to work on the mushroom sector after inspections between 2007 and 2010 found workers being exploited at 25% of the firms they inspected.

Between 2011 and 2014, officials visited 187 growers and agencies and found issues at 20%. In total, 236 workers were discovered without permits, most of whom came from Bulgaria. In addition, 193 workers – mainly Bulgarian and Polish nationals – were being paid below official rates.

Mushroom growers were fined a total of €5m for employing illegal immigrants and underpaying staff. In addition, the tax office issued demands for €3m in fines and unpaid tax bills at 20 different companies.

Most mushroom farms are found in Limburg, Brabant and Gelderland. The number of firms involved in growing mushrooms has shrunk from 517 in 2000 to 123 last year. This is partly due to consolidation, the social affairs ministry said in a statement.

It hopes the introduction of a Fair Produce labelling system will encourage growers to pay workers properly.