A study by the universities of Sheffield and Bath found that while companies can increasingly trace where their products come from, many do not know the backgrounds of their staff.
Researchers interviewed business experts, NGOs, trade unions, law firms and the police, and found that layers of outsourcing, subcontracting and the informal hiring of temporary staff enabled victims of slave labour to be hidden in the workforce.
Earlier this year, figures showed the number of suspected victims of slavery and human trafficking has more than doubled in three years. The study, which focused on construction and food companies, found that most examples of forced labour were several steps removed from the core workforce.
Within the agricultural sector, these employees could potentially only be on site for days or weeks, making it difficult for companies to detect abuse.
Dr Genevieve LeBaron, from the University of Sheffield’s Department of Politics, said: “Leading UK companies are starting to belatedly wake up to the fact that their existing systems for detecting worker abuse simply are not fit for purpose for uncovering forced labour.
“But, as new initiatives emerge, the critical factor determining their success will be whether they meaningfully address the labour supply chains that feed their business.
“It is these chains that make forced labour seemingly invisible even when the workers subjected to them are right in front of us in the farms, factories and construction sites that surround our communities.”
Professor Andrew Crane, of the University of Bath, said companies had “little hope” of detecting modern slavery practices unless they focused specifically on their labour supply chains.
“Twenty years ago most high street retailers did not have a clue where the products they sold actually came from,” he said.
“Since then, there has been a revolution in responsible business practices and companies have invested millions of pounds to trace the source of their products and tackle the myriad sustainability issues they found there.
“To prevent the misery of modern slavery from blighting our workforces companies must apply that same focus to their staff.”
The study found that companies believed they were able to protect themselves from modern slavery because of the investment they had made in responsible product sourcing.
But the researchers say this focus on tracing product supply does not equip companies to easily trace where their workers have come from, or the types of exploitation they have been exposed to.
One UK hotel chain chief executive told the study:
“I can tell you the farm where the steak on your plate came from, probably even the name of the cow.
“But we have no idea where the workers came from that work in our kitchens.”