Rishi Sunak’s immigration bill was heavily criticised by senior Conservative MPs as the government overturned amendments made by the House of Lords. Theresa May and Tim Loughton were among more than a dozen backbench Tories seeking further changes to the illegal migration bill, which the prime minister says is crucial to stopping small boats from crossing the Channel. On Tuesday, they sought further protections for people claiming to be victims of modern slavery, pregnant women and children who arrive in the UK via small boats. MPs voted on numerous amendments made by the Lords to the contentious bill. In a session involving 18 separate formal votes, the Lords’ amendments were all overturned – despite a small group of Tory rebels fighting to keep the peers’ proposals on unaccompanied children, safe and legal routes, and modern slavery protections. The voting in the Commons lasted nearly four hours. The Home Office offered several concessions on Monday evening, including on time limits for the detention of children and pregnant women as well as removing a clause so the law, if enacted, would no longer apply retrospectively from when it was first announced in March. May was among those seeking further changes. The former prime minister said the bill “will consign more people to slavery”, adding she would have to “persist in disagreeing with the government”. “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,” she said. “Not stop false claims of modern slavery, but stop all claims full stop and that is where I depart from the government.” May cited a scenario of a young woman trapped “in slavery”, warning that under the proposed bill “she would get no support”. “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’ve been in a living hell, we don’t care that you have been the victim of crime,’” she said. Loughton, the former children’s minister, told the Commons: “Only last night at about quarter to eight did the government publish their amendments, which we had just a few hours to scrutinise ahead of today. “This needs proper explanation, it has not been properly explained, the assurances that we were promised have not materialised or, if they have, I am afraid nobody understands them. “So I hope this house will make sure they go back to the Lords in order for further concessions to be given.” David Simmonds said he would be following May in trying to preserve the UK’s modern slavery laws. The Tory MP for Ruislip, Northwood and Pinner told the Commons: “I remain concerned about a number of aspects of how this bill would operate in the real world.” Earlier, the former home secretary Priti Patel tweeted: “We were told that the illegal migration bill would ‘stop the boats’. Key pillars of that bill have now been abandoned.” She also said her former department was now spending £500,000 a day on 5,000 empty hotel beds as a buffer for higher-than-expected numbers of people crossing the Channel. More than 13,000 people have crossed the Channel in small boats to the UK so far this year, including more than 1,600 in the last four days, the latest government figures show. Opening Tuesday’s debate, Robert Jenrick said some of the Lords’ changes were “little short of wrecking amendments”. “The ‘stop the boats bill’ is designed to ensure the UK can be an even greater force for good in the world by using our finite resources on those who truly need it,” the immigration minister said. The bill will go back to the Lords on Wednesday during so-called parliamentary ping-pong, when legislation is batted between peers and MPs until agreement on the wording can be reached. It will return to the Commons on Monday before then going back to the Lords next Tuesday.