Anti-slavery commissioner Kevin Hyland has contacted a quarter of FTSE 100 companies calling for stronger action to combat modern slavery.
Hyland has written to 25 firms identified as non-compliant with the Modern Slavery Act (MSA) regarding their reporting of efforts to address slavery in supply chains.
Under the MSA firms with a turnover of £36m or more must produce slavery statements approved by the board, signed by a director and published on a website with a link from the homepage.
Research by the Business and Human Rights Resource Centre (BHRCC) last year found a quarter of FTSE 100 firms had not met these three minimum requirements, and had still not done so by the end of 2017.
Hyland said: "Despite some encouraging, positive change since the legislation came into force, 2016’s corporate modern slavery statements were patchy in quality, with some companies failing to produce them at all and others demonstrating little meaningful engagement with the issues."
Some 40m people are estimated to be enslaved today, of which 16m are thought to be working for companies around the world.
"FTSE 100 companies have a major influence in eradicating modern slavery," said Hyland. "Therefore, I have written to 25 companies identified in the BHRCC research as non-compliant, and which had still not corrected their omissions by December 2017, to encourage improved efforts in the coming year.
"Taking action on modern slavery and human trafficking is not just a moral obligation – it is in fact good business sense: forced labour in company operations or supply chains has potential to disrupt business, weaken investor confidence, incur litigation costs and cause significant brand damage."
The commissioner, in collaboration with the Local Government Association, has also produced guidance for procurement professionals working in local councils to help them tackle slavery.
This includes advice on mapping supply chains, engaging with suppliers and collaborating with other councils to reduce costs.
The report quotes Parliament's Joint Committee on Human Rights: "If the government expects business to take human rights issues in their supply chains seriously, it must demonstrate at least the same level of commitment in its own procurement supply chains."
The BHRCC research, from October 2017, commended Marks & Spencer, Sainsbury's, Unilever, British American Tobacco, Tesco and Vodafone for their work against modern slavery. Hargreaves Lansdown, Paddy Power Betfair, Pearson and Worldpay rated poorly.