More than half of UK police forces are handing over victims of crime to the Home Office for immigration enforcement, new figures show.
One woman who was beaten by her partner was then herself arrested by police.
There are fears the approach is stopping vulnerable people - including rape victims - reporting crimes, playing into the hands of traffickers.
The Home Office said it would support vulnerable migrants "regardless of their immigration status".
"Victims of crime must be treated first and foremost as victims," a spokesperson said.
Police force replies
The BBC's Victoria Derbyshire programme asked 45 UK police forces, via a Freedom of Information request, if they referred victims and witnesses of crime to the Home Office for immigration enforcement.
Twenty-seven said they did. Some gave a straight "yes", others had caveats such as "not routinely" or "it's rare".
Three - including Police Scotland - said they did not, and the rest were unclear, did not reply or said they had no data.
"Sara" came to the UK with her partner - a British citizen. But she says she was treated like a slave.
"He told me, 'That's why I brought you here, so you can cook and clean for me,'" she explains.
"He beat me with a belt and a cable."
She was brought into the UK illegally, so she could not go to the police in case she was arrested - "a common feature in all domestic violence and trafficking cases", according to Sara's lawyer Sulaiha Ali, of Duncan Lewis Solicitors.
Eventually, Sara tried to escape.
She ran out on to the street, when her partner chased after her and beat her in front of a member of the public, who then called the police.
They arrested the perpetrator and took Sara to hospital because of the severity of her injuries.
She was then taken to a hostel, where she was later arrested and sent to Yarl's Wood immigration removal centre to be deported.
Ms Ali thinks Sara should never have been arrested at all.
"It's shocking to know that victims of crime are being seen and treated as criminals just because of their status."
Ms Ali has now stopped Sara's deportation order, and Sara has applied for asylum in the UK.
But Ms Ali says she's doing "quite bad", and has not been given the support "she is entitled to" as a victim, because "the focus has been completely on her immigration status".
Pragna Patel from Southall Black Sisters - which campaigns on the issue - says she is extremely worried that referring victims of crime for immigration enforcement is "in conflict with the government's stated aim to protect all women from violence".
"Since 2014, we've seen a steady rise in cases where the police have arrested women or reported women to the Home Office as potential illegals rather than deal with their reports of violence and rape."
She fears vulnerable women will be deterred from speaking out about the violence and abuse they have suffered because they are frightened of being arrested, detained and deported.
Green Party leader Caroline Lucas told the Victoria Derbyshire programme she has heard of rape victims "being afraid to come forward to report that rape, which means that the perpetrator is still at large".
She called for a "firewall" - a blocking of information between police operations and immigration officials - so the two do not become mixed up, and justice is not "jeopardised".
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Last November, a case was uncovered in which a woman reported to the police that she had been kidnapped and raped over a six-month period.
She was taken to a sexual assault centre by police, but then she was arrested.
Guidance from the National Police Chiefs Council (NPCC) issued in December says that "immediate arrest will not be made" of victims of crime, relating to their immigration status, unless there is "an immediate risk of harm to a specific individual".
Only three police forces said they were following this guidance.
Former Ch Supt Dal Babu has called for more specific regulations for officers to follow.
He said victims of crime "were low-hanging fruit" amid the government's hostile environment policy, which included immigration removal targets.
"These are vulnerable people... so it's much easier when a woman comes forward who has been raped to then say, 'We're investigating this', and go and arrest [her] and [she'll] be sent to a detention centre."
The NPCC said it was "unequivocal that victims of crime should be treated as victims first and foremost.
"Each case is considered very carefully but there will be instances where police need to exchange information with the Home Office."
The Home Office said: "When individuals are found to have no basis in the UK, we carefully consider the details of the case before taking an enforcement action."
Additional reporting by Leo Sands.
Watch the BBC's Victoria Derbyshire programme on weekdays between 09:00 and 11:00 on BBC Two and the BBC News channel.