Video report from ITV News National Editor Allegra Stratton
Police are failing thousands of victims of modern slavery in the UK by "substandard" reporting of the crime, it has been warned.
The UK's Independent Anti-Slavery Commissioner called for "urgent improvements" to the UK National Referral Mechanism (NRM), the framework for identifying and supporting victims.
Kevin Hyland criticised "chronic weaknesses" and said unless police response to modern slavery "dramatically improves", victims "both present and future are being failed".
A total of 884 modern slavery crimes were recorded by police in England and Wales over the 2015/16 financial year, but 3,146 referrals were made to the NRM during the same period.
Up to 13,000 people are held as slaves in the UK, where they are "oppressed as domestic slaves, abused as agricultural workers and exploited in brothels", the report stated.
Police forces with the most slavery crimes recorded in 2015/16
1) Metropolitan Police - 248
2) West Midlands - 89
3) West Yorkshire - 83
4) Greater Manchester - 80
5) Kent - 29
= Essex - 29
Although it is not possible to directly link specific NRM referrals to recorded crimes using the data, analysis indicates that at best 28% of referrals may have resulted in a modern slavery crime being recorded by police in England and Wales, according to the report.
Mr Hyland said some forces are taking a "proactive approach" to combating modern slavery but "many instances of substandard modern slavery crime recording remain".
Twenty-one police forces in England and Wales were either unable to answer whether a referral that their service had made to the NRM had resulted in a modern slavery crime record, or expressed "significant challenges" to doing so. Four constabularies could not find any internal record of the NRM referrals made by their force.
Meanwhile, the Police Service of Northern Ireland reported that 100% of NRM referrals in 2015/16 were "crimed" under a human trafficking and exploitation offence.
Speaking to ITV News, Mr Hyland said:
In 2015, 117 people were prosecuted for modern slavery offences in the UK, 19% higher than the 98 prosecuted in 2014. The report said while this is "encouraging", the number of individuals being brought to justice is still "relatively low given the estimated scale of offending".
Describing modern slavery as a "gross injustice", Mr Hyland added: "The children working in the mines of the Democratic Republic of Congo to produce cobalt for smartphone batteries, the Eastern European men exploited in shocking conditions in car washes across the UK, the young girls trafficked to work as tea-pickers in the fields of Assamt, the domestic slaves abused in wealthy London residences, the Nigerian women and girls trafficked across the Sahara to work as sex slaves in Europe - these are all individuals. Someone's mother, father, brother, sister, daughter or son - with a freedom and a future that must be fought for."
Cardinal Vincent Nichols, the head of the Roman Catholic Church in England and Wales, has set up a joint clergy and police group to tackle human trafficking.
He told ITV News victims are not just from parts of the world typically associated with people trafficking, such as eastern Europe, but also from Britain.
"Human trafficking is a vast international evil network - we have to put other international networks against it and the churches have that capacity," Cardinal Vincent said.
Shaun Sawyer, National Police Chiefs' Council (NPCC) lead for modern slavery, said police are determined to play their part in eradicating the "vile crime".
"This complex and cross-border type offence presents the police with unique challenges," he said. "However, it does not excuse the gap between national referrals and recorded crime in some areas.
"We also believe there are many victims of modern slavery who are yet to be identified."
The Modern Slavery Act, the first legislation of its kind in Europe, took effect last year.