Slave drivers and human traffickers have been allowed to carry on offending unchecked because of police failings, a watchdog report has warned.
Cases are being shelved prematurely, investigations delayed by several months and clear signals of crimes missed, inspectors found.
As a result of the shortcomings, those suffering at the hands of perpetrators were not always recognised as victims.
Instead they remained in the hands of those exploiting them or were arrested as offenders or illegal immigrants, according to the assessment from HM Inspectorate of Constabulary and Fire and Rescue Services (HMICFRS).
It also found there was "variable commitment" among police leaders to tackling human trafficking and modern slavery, which are thought to affect tens of thousands of people in the UK.
Some senior officers openly expressed a reluctance to "turn over the stone" and proactively look for offences in the categories, citing concerns about the "potential level of demand", the report said.
HM Inspector of Constabulary Wendy Williams, who led the probe, said police have a "crucial role" to play in protecting thousands of men, women and children who are being "degraded and dehumanised" every day.
She said: "Whilst modern slavery cases can be complex and require significant manpower, many of the shortcomings in investigating these cases reflect deficiencies in basic policing practice.
"As a result, victims were being left unprotected, leaving perpetrators free to continue to exploit people as commodities."
Modern slavery and human trafficking can cover a wide range of offending, including forced labour, sexual exploitation and domestic servitude.
An official estimate previously suggested there are up to 13,000 potential modern slavery victims in the UK, but last week the anti-slavery commissioner described this figure as "far too modest".
Premises targeted as part of police activity include nail bars, brothels and car washes.
The HMICFRS report said victims are being let down "at every stage".
Failings included: a clear tendency to close cases prematurely, sometimes without speaking to victims or witnesses; extensive delays of up to eight months in starting investigations; and a lack of focus on safeguarding all potential victims.
In one case a woman was forced to work as a prostitute but no rape offences were recorded despite her reporting her experience to police.
In another immigration authorities contacted a force about a victim of domestic servitude trafficked into the country by private jet - but no investigation was started to identify suspects or locations of interest.
The report also raised concerns that potential victims were identified and treated from the outset primarily as immigration offenders.
Shaun Sawyer, the national policing lead for modern slavery, said: "We fully accept the recommendations included in this report. The police service is now actively seeking out and uncovering modern slavery.
"Across England and Wales there are currently over 400 active investigations, an increase of 218% from November 2016 - 85% of which are led by the police."
A Home Office spokesman said the Government is investing £8.5 million to help the police tackle modern slavery, describing it as a "barbaric" crime.