Modern-day slaves are being failed by "substandard" crime recording, the UK's anti-slavery commissioner has warned.
When referrals are made to the National Referral Mechanism (NRM) - a framework for identifying potential victims - and are not recorded by the police, investigations are not launched and victims do not receive the justice and support they need and deserve, Kevin Hyland warned.
Figures compiled for the commissioner's annual report show 884 modern slavery crimes were recorded by police in England and Wales in 2015/16 - but in that same period 3,146 referrals were made to the NRM.
Hyland criticised "chronic weaknesses" as he demanded that law enforcement agencies step up their response to the issue.
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.
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.
Some forces were able to identify reasons why referrals had not resulted in a modern slavery crime record, but several of these were "unacceptable".
The number of modern slavery crimes recorded by forces in 2015/16 ranged from zero to more than 200, data showed.
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.
However, the commissioner warned that the number of individuals being brought to justice is still relatively low.
Hyland said: "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 Assam.
"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."
Hyland's report said: "Northern Ireland's success demonstrates that it is possible to record modern slavery crime accurately. As the commissioner's findings have uncovered, however, chronic weaknesses in modern slavery crime recording in England and Wales remain.
"Inadequacies in this area impact not only present and future victims, but could also allow organised crime groups to act with impunity, compromising the UK's national security."
In 2015, referrals of potential victims to the NRM increased by 40%, while the number of modern slavery prosecutions has risen.
Shaun Sawyer, National Police Chiefs' Council (NPCC) lead for modern slavery, said police are determined to play their part in eradicating the "vile crime".
He said: "This complex and cross-border type offence presents the police with unique challenges. 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.
The Government has earmarked £33 million from the UK aid budget to focus on high-risk countries from which victims are regularly trafficked to the UK, while Theresa May has established a new taskforce on modern slavery.
The Prime Minister said: "So let us send out this message. To the victims of modern slavery: we will not ignore your plight. We will not turn away. We will not shut our eyes and pretend your suffering does not exist.
"And my message to these criminals is simply this: we are coming after you."
Home Secretary Amber Rudd said the Government has taken "world-leading action" to tackle the "barbaric" crime.