Theresa May has attacked the government’s Illegal Migration Bill, saying it would “enable more slave drivers to… make money out of human misery” and “consign more people to slavery”.

The former prime minister, who has long campaigned on tackling modern slavery in the UK, said ministers wanted to use the new legislation to “stop the boats” crossing the English Channel.

But in a passionate speech in the Commons, she warned the bill needed to change or would lead to victims getting no support – and perpetrators getting away with their crimes.

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MPs have been debating – and are now voting on – proposed amendments to the Illegal Migration Bill made by peers after 20 defeats were inflicted on the legislation in the House of Lords.

The bill itself aims for the “swift detention and removal” of people arriving in the UK illegally, sending them back to their country of origin or a third country – like Rwanda.

But peers had a number of issues with the legislation – especially around the treatment of women, children, people from the LGBT+ community, and victims of modern slavery – and sent back a number of changes for the Commons to look at.

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The government announced its own fresh amendments on Monday in an attempt to stop MPs supporting the Lords, including a promise that new rules on removing people would not be applied retrospectively – only after the bill becomes law.

They also reduced the time children could be detained for – eight days instead of the proposed 28 – and pledged to keep the current rule on detaining pregnant women for a maximum of 72 hours.

But the concessions have not gone far enough for some senior backbenchers, and have led to terse exchanges in the Commons from the government’s own benches.

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Mrs May welcomed some of the changes brought forward by the Home Office, “whatever the motivation”, but added: “I want support to continue for the victims of modern slavery in the UK after commencement of the bill.”

She pointed to amendment 56 from the Lords, which seeks to ensure victims will not be detained and removed from the UK if they have been modern slaves, and urged the government to support it.

“This bill has been marketed as a ‘stop the boats’ bill,” she said. “We all want to stop the boats, nobody wants to see people risking their lives in small boats across the Channel.

“But this bill is not just written to stop the boats. It covers all illegal migration. And its unwritten subtext is the ‘stop certain victims’ claims of modern slavery’ bill, not stop false claims of modern slavery, but stop all claims full stop, and that is where I depart from the government.”

The former PM gave the hypothetical situation of a woman “persuaded” by a man to come to the country “for a great job and wonderful life together”, ending up trafficked into prostitution and not even knowing the papers he used to get her in were illegal.

“Under this bill, she would get no support,” said Mrs May. “The government’s response would be, ‘we don’t care that you’ve been in slavery in the UK, we don’t care that you have been in a living hell, we don’t care that you have been the victim of crime.

“‘We do care that you came here illegally, even though you probably didn’t know it, so we are going to detain you and send you home, even if it is into the arms of the very people who trafficked you here in the first place, or we want to send you to Rwanda’.”

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Mrs May also warned the bill “ties the hands of the police and it undoes the good work of the Modern Slavery Act”, as it would stop victims providing evidence to catch the perpetrators.

“It will enable more slave drivers to operate and make money out of human misery,” said the influential MP. “It will consign more people to slavery. No doubt about it, I think if [the Lord’s amendment] is overthrown, that will be the impact.”

MPs have begun voting on the amendments from both peers and the government and will continue throughout the evening, before the bill heads back to the Lords for another round of scrutiny.

And Mrs May made clear where she stood.

“The government wants to deny certain victims of modern slavery support. It will deeply damage the operation of the Modern Slavery Act,” she added.

“The alternative is to let [the Lord’s amendment] stand. If the government persists in disagreeing… then I will have to persist in disagreeing with the government.”

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