Britain's government Monday enacted new measures to accelerate the deportation of foreign criminals, clamping down on some who have claimed protection under UK law as purported victims of "modern slavery".

It cited the case of one convicted rapist who appealed against a decision by the Home Office (interior ministry) to expel him from Britain, by claiming he was a victim of criminal gangs engaged in human trafficking.

He was bailed pending the appeal, committed another rape, and remains in the UK, the Home Office said.

"It is totally unfair that genuine victims of modern slavery may be left waiting longer to receive the protections they need due to the flagrant abuse of the system," Home Secretary Suella Braverman said in a statement.

"The changes coming into force will mean if you've committed an offence, we have the power to refuse your protections and kick you out of our country," she said.

The measures taking effect under a new Nationality and Borders Act mean that Home Office caseworkers can in future demand evidence of modern slavery, rather than taking a victim's word.

That could include evidence from a charity worker or police officer who has helped rescue the victim.

But the changes have been criticised by rights groups for undermining protections for genuine victims. One Braverman initiative -- to fly cross-Channel migrants to Rwanda for permanent resettlement -- has already been blocked in the courts.

Britain's National Crime Agency reported in November that Albanian crime groups in particular were manipulating the National Referral Mechanism (NRM), which is being reformed under the new act.

Established in 2009 to help protect human trafficking victims, the NRM is used to identify and refer them to UK government agencies to ensure they receive appropriate support.

If caught working in cannabis farms or other criminal enterprises, Albanian migrants have been coached to claim they are victims of modern-day slavery and apply to the NRM, the crime agency's report said.

In December, Prime Minister Rishi Sunak announced a new deal with Albania to stem the flow of migrants from the country crossing the Channel on small boats from mainland Europe.

The agreement only came about after the government in Tirana demanded an apology for an anti-migrant "campaign" in UK media, following some incendiary rhetoric from Braverman.

But the minister has maintained a hard line, to the delight of Conservative right-wingers keen to show that Britain can control its borders after Brexit, and the clampdown is one of five priorities promised by Sunak for this year.

"We must stop people exploiting our immigration and asylum laws," Braverman said in Monday's statement.

"And I am personally determined to crack down on those abusing the generosity of the British public and taking our country for a ride."