Former prime minister Theresa May has criticised Rishi Sunak’s small boats crackdown – warning that the new asylum bill “shuts the door” on genuine victims of modern slavery.

Raising doubts about the PM’s promise to “stop the boats”, the former Tory leader said anyone who believed the legislation would halt “illegal migration once and for all is wrong”.

Ms May condemned the “blanket dismissal” of anyone facing persecution who arrives by authorised routes, telling the Commons: “By definition, someone fleeing for their life will more often than not be unable to access a legal route.”

Ms May also raised her concerns about genuine trafficking victims – telling MPs that Suella Braverman’s Illegal Migration Bill would “remove support from the victims of trafficking and modern slavery”.

She added: “As it currently stands, we are shutting the door to victims who are being trafficked into slavery into the UK. The Home Office knows this bill means that genuine victims of modern slavery will be denied support.”

Conservative MPs were set to vote through Illegal Migration Bill at the second reading stage on Monday night – but a growing number of backbenchers have made clear their opposition to current plans.

Caroline Nokes became the first Tory MP to say they could not vote for the plan to detain and deport small boats arrivals – saying the bill gives her “absolute horror”.

The women and equalities select committee chair she was “deeply troubled” at the prospect of a policy “criminalising children and pregnant women” after it emerged that children coming across the Channel will be detained in immigration centres.

Tory MP Chris Skidmore also said he could not vote for the bill that breaks international law “or the human rights conventions that the UK has had a proud history of playing a leading role in establishing”.

Senior Tory MP Simon Hoare said many colleagues would be voting for the bill on Monday only on “good faith tonight in the expectation that the bill can be amended.”

The Northern Ireland select committee chair – from the moderate ‘one nation’ wing of the party – said amendments would be needed to offer greater protection to children and women who are trafficked.

Former justice secretary Robert Buckland – who has told The Independent it was “not right” to lock up children – told the Commons the bill “is not yet in the state it needs to be” and said he hoped it would not be used as “battering ram” against the European Convention of Human Rights.

Labour’s shadow home secretary Yvette Cooper acknowledged some Tory MPs were “deeply troubled” by aspects of the bill. “I think it reflects quite how far the Conservative Party has fallen.”

Ms Cooper said the bill was a “con that makes the chaos worse” and “will lock up children”. She added: “It won’t return everyone, in fact it makes it harder to get return agreements. It won’t clear the asylum backlog, in fact it will mean tens of 1000s more people in asylum accommodation and hotels.”

Ms Braverman defended the bill as way of preventing people coming across the Channel from “overwhelming our asylum system” – before adding that Britain has seen “too much” immigration in general in recent years.

The home secretary said it was “perfectly respectable for a child of immigrants like me” to say “we’ve had too much of it in recent years and to say that uncontrolled and illegal migration is simply bad”.

She accused some of her critics of “grotesque slurs”, adding: “The worst among them, poisoned by the extreme ideology of identity politics, suggest that a person’s skin colour should dictate their political views.”

Claiming her plans were “backed by the majority of the British people”, Ms Braverman added: “I will not be hectored by out-of-touch lefties … And I will not back down when faced with spurious accusations of bigotry.”

One former Tory minister told The Independent the plan to allow child detention was “sickening” – fearing it will give immigration officers the power to restrain minors, as well as open up the possibility of their removal on deportation flights.

The Refugee Council has said the legislation allows the deportation of unaccompanied children if returning them to their country of origin is deemed safe – an idea downplayed by the home secretary.

Ms Braverman said “only in limited circumstances, such as for the purposes of family reunion, we will remove unaccompanied asylum seeking children from the UK”.

She said the focus would be on removing adult men under the age of 40. “But we must not create incentives for the smugglers to focus on people with particular characteristics by signposting exemptions for removal,” she add