Children are among hundreds of potential victims identified in a major police crackdown on forced work, domestic servitude and sexual exploitation.
Since May last year, forces have launched 200 operations into modern slavery, targeting around 900 alleged offenders.
Forty children were among 1,689 potential victims detected. The number was made up mainly of adults, who are often vulnerable through homelessness, isolation, or drug or alcohol dependency.
The figures were disclosed by police chiefs as forces step up activities to tackle modern slavery, which has been described by Theresa May as a "scourge on our society".
Labour exploitation featured frequently in the drive, with 93 probes into people being forced to work against their will in construction, car washes or factories.
Sixty operations targeted sexual exploitation - predominantly forced prostitution within brothels, and 14 were against domestic servitude.
Twelve were against child sexual exploitation and four were for sham marriages, while some involved forced cannabis cultivation and shoplifting.
Over half of the operations involved alleged offenders and victims from either Romania or Britain, the National Police Chiefs' Council said. Cases also involved suspects and victims from Poland, Lithuania and Vietnam.
The 200 operations have led to 20 convictions and 24 charges so far, with other investigations ongoing.
In 2013, research by the Home Office estimated that there were between 10,000 and 13,000 potential victims of modern slavery.
NPCC Lead for Modern Slavery Shaun Sawyer said the number could be "far higher, particularly if we include young people in the UK who are exploited and forced to work in drug dealing operations".
He said: "Modern slavery investigations are fraught with complexity and challenge, and testimony from victims is hard won.
"Some victims do not see themselves as such. Bringing cases through the criminal justice system can take several years, demanding significant police resources."
Mr Sawyer said most operations were instigated after a tip-off from a member of the public about "visible signs of exploitation" such as anti-social behaviour, overcrowding or squalid living conditions or indicators of prostitution.
These reports are vital, he said, urging anyone with concerns to contact authorities.
Last month, the UK's anti-slavery commissioner warned that victims were being failed by "substandard" crime recording which risks allowing perpetrators to offend with impunity.
Kevin Hyland criticised "chronic weaknesses" as he demanded that law enforcement agencies step up their response to the issue.
The Government has earmarked £33 million from the 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.
Home Secretary Amber Rudd has also announced an £8.5 million cash injection to bolster police efforts.
Mark Burns-Williamson, police and crime commissioner for West Yorkshire and chairman of the National Anti-Trafficking and Modern Slavery Network, said the trafficking of people and modern slavery are "abhorrent abuses of human rights".
He added: "These crimes are a major challenge for all of us, but there is a commitment to tackle these issues and prevent harm and suffering."