The PM’s current Rwanda plan will mean no asylum seekers are flown there before the next election, sacked home secretary Suella Braverman has said.
Writing in the Telegraph, she said “tinkering with a failed plan” would not achieve the government’s aims.
She said the government should ignore human rights laws and obligations in their “entirety” to push it through.
Rishi Sunak plans emergency legislation to rescue the policy after the Supreme Court ruled it unlawful.
Hours after their decision, he announced plans for the new legislation and a new treaty with Rwanda, so that first flights could begin in the spring.
But Mrs Braverman said a new treaty was “magical thinking” and would not solve “the fundamental issue” – that the UK’s highest court had decided there was a risk Rwanda could return asylum seekers to countries they had fled from, putting them at risk of harm.
In its ruling, the Supreme Court said there were “substantial grounds” to believe those deported to Rwanda could be sent back to places where they would be unsafe.
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In her first detailed response to the ruling, Mrs Braverman, who was sacked as home secretary on Monday, said that unless the prime minister went further than his current proposals she could not see how the government could deliver on its pledge before running out of Parliamentary time.
“Any new treaty would still require going back through the courts, a process that would likely take at least another year,” she said.
She added that the process “could culminate in yet another defeat”,
“That is why the plan outlined by the PM will not yield flights to Rwanda before an election if Plan B is simply a tweaked version of the failed Plan A,” she said.
A general election is expected to be held next year and one must be held by January 2025.
Mrs Braverman said the PM’s proposed legislation should ignore “the entirety” of the Human Rights Act and the European Convention on Human Rights (ECHR), as well as other relevant international obligations including the Refugee Convention.
She added that the UK must take practical steps to improve Rwanda’s asylum system, for example by embedding UK observers or independent reviewers of asylum decisions there.
Legislation must also make clear those arriving in the UK illegally would be detained until removal.
Mrs Braverman added that a bill should be introduced by Parliament’s Christmas recess and MPs should be recalled to sit and debate it over the holiday.
In language tamer than her recent public interventions, she said there was “no reason” to criticise Supreme Court judges for their ruling.
Instead, she blamed “the politicians who have failed to introduce legislation that would guarantee delivery of our Rwanda partnership”.
In a scathing letter after her sacking, Mrs Braverman accused the prime minister of “magical thinking” and betraying his promise to stop small boat crossings.
The prime minister says the new treaty would guarantee that Rwanda would not send migrants back to countries where they could be persecuted or harmed. He is also proposing legislation to certify that Rwanda is a “safe” country – despite the Supreme Court’s findings.
The Rwanda policy is central to Mr Sunak’s plan to stop asylum seekers crossing the Channel in small boats – one of his key pledges – as it is designed to deter people from making the dangerous journey.
Earlier, Chancellor Jeremy Hunt said the government was “hopeful” Rwanda flights would take off next year but that he could not “guarantee” this.
Downing Street has said legislation would be produced “in the coming weeks”.
However, it is expected to face opposition in the House of Lords and many are also anticipating further legal challenges in the courts.
Transport Secretary Mark Harper told BBC Breakfast that the government was “committed” to getting the Rwanda policy working by the spring.
Sir David Normington, former Home Office permanent secretary, told Radio 4’s Today programme that Mrs Braverman was “right in one way” – that getting a working Rwanda policy “would be very difficult”.
“We could pull out of all conventions but that would be a very bad idea,” he said, adding that it would always come down to a British court deciding whether Rwanda was safe.
“The courts say it is not a safe country. You can’t say black is white.”
Asked if international law was “outdated”, Sir David said that “at the core” international agreements were written to protect the vulnerable.
“What is true is that the rights of people to not be tortured never goes out of date.”
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Analysis box by Dominic Casciani, home and legal correspondent
It’s not immediately clear how Mrs Braverman’s plan would legally work quickly.
The UK and other countries that are signed up to the European Convention on Human Rights can only put to one side some of its protections in times of war or some other emergency. The key protection at the heart of the Rwanda case – that nobody should be subjected to torture or to inhuman treatment – is not one of the rights that can be swept away in what’s known as “derogation”.
The UK has only derogated from the ECHR eight times in 70 years. Seven of those situations were related to detaining paramilitaries during the conflict in Northern Ireland. The most recent in 2001 concerned holding al-Qaeda suspects without charge – a move that the courts later said was illegal.
During Boris Johnson’s time as prime minister, the government proposed limiting and replacing some human rights protections in a highly-criticised replacement bill which Rishi Sunak then scrapped.
Leaving the ECHR entirely would separately breach the 25-year-old Good Friday Agreement at the heart of Northern Ireland’s power-sharing peace deal – and enrage the UK’s partners on the other side of the English Channel – potentially making co-operation on stopping boats harder.