Suella Braverman has admitted the government is attempting to push “the boundaries of international law” with legislation aimed at reducing small boat crossings in the Channel. The law, to be disclosed to MPs at lunchtime on Tuesday, is expected to place a legal duty on the home secretary to detain and remove nearly all asylum seekers who arrive “irregularly” such as via small boats in the Channel. The law will be backdated to allow the removal of anyone who reaches the UK from the moment the home secretary speaks to MPs, in an acknowledgment that refugees may try to cross before the law is passed. On Monday night Rishi Sunak argued that his new bill, which is key to one of his five priorities for his premiership, will “take back control of our borders, once and for all”. There will be constraints on the rights of migrants to use a judicial review to challenge decisions, as ministers attempt to bypass the legal wrangles that have prevented the implementation of plans to send people to Rwanda. But critics including former Tory ministers, Labour and refugee charities have warned the proposals are unworkable and will leave thousands of people in limbo by banning them from ever claiming British citizenship again. There are few returns agreements with other countries in place, despite more than 80,000 people expected to arrive in the UK via small boats this year. There is at present limited capacity to detain several thousand asylum seekers. Despite plans such as forcibly removing asylum seekers to Rwanda being mired in legal challenges, ministers were expected to approach the limits of the European convention on human rights with the new legislation. There is expected to be an admission that the legislation may not be compliant with the convention, although Home Office sources believe it does accord with international law. Writing in the Daily Telegraph, Braverman said: “We must stop the boats and that’s what our bill will do. No more sticking plasters or shying away from the difficult decisions. “Myself and the prime minister have been working tirelessly to ensure we have a bill that works – we’ve pushed the boundaries of international law to solve this crisis. “If you come here illegally it must be that you cannot stay.” Sunak told a cabinet meeting on Tuesday morning that the new law was “tough, fair and necessary action”, according to an official readout. Speaking later, Sunak’s official spokesperson said the expectation was that elements of the plan were likely to be challenged in the courts. While there is a parallel pledge to create new legal asylum routes into the UK, Sunak’s spokesperson said these would only begin “once we have stopped the boats”, to mitigate the pressures on local authorities and others dealing with arrivals. “Opening up more, whilst at a time not having clear sight on how many will come here illegally is, we do not think, a sustainable approach,” he said. A duty will be placed on the home secretary to remove “as soon as reasonably practicable” anyone who arrives on a small boat, either to Rwanda or a “safe third country”. Arrivals will be prevented from claiming asylum while in the UK, with plans also to ban them from returning once removed. Sunak spoke to Rwanda’s president, Paul Kagame, on Monday night before unveiling the proposed law, and promised to continue working with him to ensure their stalled project works. The government has paid more than £140m to Rwanda but no flights forcibly carrying people to the capital, Kigali, have taken off because of legal challenges. A Downing Street spokesperson said: “The leaders committed to continue working together to ensure this important partnership is delivered successfully.” The prime minister will meet France’s president, Emmanuel Macron, on Friday to discuss further cooperation that will be required to reduce boat crossings. Sunak admitted voters “have heard promises before” without seeing results, but insisted his legislation “will mean that those who come here on small boats can’t claim asylum here”. He wrote in the Sun: “This new law will send a clear signal that if you come to this country illegally, you will be swiftly removed.” The Immigration Services Union representing border staff said the plans were “quite confusing” and did not seem possible without the Rwanda policy functioning. Lucy Moreton, the union’s professional officer, also suggested on BBC Radio 4’s Today programme that smuggling gangs would tell people “quick, cross now before anything changes”, risking an increase in the number of crossings. The Labour leader, Keir Starmer, raised doubts about the legality and feasibility of the plans after the last attempt failed “to get us very far”. The prime minister has made “stopping the boats” one of his five priorities and has been under pressure to tackle the issue amid dire polling figures for the Tories. Almost 3,000 people have made unauthorised crossings of the Channel already this year. The Refugee Council chief executive, Enver Solomon, said the plans “shatter the UK’s longstanding commitment under the UN convention to give people a fair hearing regardless of the path they have taken to reach our shores”. He added: “The government’s flawed legislation will not stop the boats but result in tens of thousands locked up in detention at huge cost, permanently in limbo and being treated as criminals simply for seeking refuge. “It’s unworkable, costly and won’t stop the boats.”