A gangmaster who kept Romanian agricultural workers in inhumane conditions in Northern Ireland while operating illegally has walked away from court with a £500 fine, provoking an outcry from rights groups and the Gangmaster Licensing Authority.

The GLA immediately announced that it would seek leave to appeal against the sentence imposed by Craigavon magistrates court on Tuesday. The chief executive of the GLA, Paul Broadbent, described the penalty as “derisory”.

Gheorge Ionas, 35, kept fellow Romanian migrants in an unheated outbuilding. Officials were told that he forced them to scavenge for out-of-date food from supermarket bins, even though he was deducting about £40 a week from their pay for food, transport and accommodation.

Ionas had arranged for the workers to travel to Northern Ireland and had paid for some of their transport by coach from Romania. He was paying them less than the minimum wage to pick apples full-time for a farmer with several orchards. One worker said he had been paid just £100 for a week’s work.

Ionas pleaded guilty to operating as a gangmaster without a licence, an offence that carries a maximum penalty of 10 years’ imprisonment.

Broadbent said he was “shocked and appalled” by the leniency of the sentence. It comes at a time when the government has been trying to develop more robust legislation to deal with modern slavery, forced labour and extreme exploitation of vulnerable workers.

“I will be writing to the public prosecutor to seek leave to appeal and express my utter dismay that slavery – for this is what this was – is seemingly not recognised in the court where the defendant appeared,” Broadbent said.

It is understood that the case was uncovered when three men, one of them employed by Ionas, presented themselves to Lurgan police station, County Armagh, asking for help.

When officers visited the gangmaster’s terraced house in Lurgan, they found three Romanian men sleeping in an outbuilding made of bare breeze blocks, which was without sanitation or heating. The migrants were classified as potential victims of trafficking because the exploitation was judged so extreme by inspectors.

They were given access to support services through the National Referral Mechanism for victims of trafficking and have since returned home.

The building was later declared unfit for human habitation by Craigavon council. Officers were told that five men had been living in it at one time.

Dr Aidan McQuade, director of Anti-Slavery International, said he was “dumbfounded” by the sentence. “That the court considered this extreme exploitation of vulnerable people worth only £500 says to others with criminal intent in this area that they don’t have much to worry about. It’s a dreadful indictment of the Northern Irish judiciary.”

The fine compares with an initial cost of £2,500 to apply for a gangmaster licence, as required by the Gangmasters (Licensing) Act.