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Boris Johnson has accepted his policy to fly asylum seekers who enter the UK illegally 4,000 miles to Rwanda for processing is likely to be challenged in the court after critics labelled the plan "evil".
He said those who enter the UK illegally on small boats or in the back of lorries will be "swiftly and humanely removed to a safe third country or their country of origin".
But, in a concession that fed into claims the policy announcement was a distraction from the partygate scandal, the PM admitted his controversial Rwanda plan was not yet ready to begin.
"I know this system will not take effect overnight," he said, "but I promise that we will do whatever it takes to deliver this new approach, initially within the limits of the existing legal and constitutional frameworks, but also prepared to explore any or further legal reforms which may be necessary."
Mr Johnson said he was "confident" about the legality of the plan "but nevertheless, we expect this will be challenged in the courts".
It came after ITV News was shown a government document raising concerns over the legality of the policy.
Responding to criticism from charities who say the plan is "cruel and nasty", the PM told a press conference: "I know there will be a vocal minority who will think these measures are draconian and lacking in passion and I simply don't agree."
An initial £120 million is expected to be given to the Rwandan government under the scheme, with Home Secretary Priti Patel striking a deal during a visit to the capital of Kigali.
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The complex has 50 rooms at present and can accommodate around 100 people with up to two people per room and sharing communal bathrooms.
But there are plans to expand the facility by building more accommodation blocks, eventually seeing it offer 150 rooms and able to sleep up to 300 people.
Asylum seekers are expected to be provided meals three times a day to eat in a communal dining room, with some kitchen facilities also available for those with special dietary requirements.
It is understood it could take up to three months to process asylum seekers.
Mr Johnson said the agreement is "uncapped" and Rwanda will have the "capacity to resettle tens of thousands of people in the years ahead", including those who have arrived "illegally" since the start of the year.
He pledged £50 million in new funding for boats, aerial surveillance and military personnel to help ensure the measures are a "very considerable deterrent" to crossings. Labour accused the prime minister of trying to distract from the partygate scandal with the "unworkable, unethical and extortionate" migration scheme.
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Party leader Sir Keir Starmer said it was an "absolutely desperate" attempt by the prime minister to "deflect and distract from his own law breaking".
Former minister Anna Soubry said on Twitter it was "distraction nonsense, inhumane & a shameful waste of taxpayers' money".
Shadow home secretary Yvette Cooper said it was an "unworkable, unethical and extortionate" plan, designed to "distract from Boris Johnson’s lawbreaking".
But Mr Johnson said action is needed to combat the “vile people smugglers” turning the Channel into a “watery graveyard”.
Many details of the expected announcement, such as whether it would apply just to those who arrived by what the government calls illegal means, remain unclear.
The document seen by ITV News which raised concerns about the plans also states that any agreement of this nature would require the government to financially incentivise whichever country it reaches a deal with.
Initial estimates had the policy costing in the tens of millions of pounds, but the document says this has been revised to the hundreds of millions of pounds.
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Similar policies have been considered in other nations like Denmark, while Australia already has a policy in place whereby asylum seekers are sent to Christmas Island for processing.
Australia has repeatedly come under criticism for this policy.
The document questions the success of these schemes, saying the evidence of effectiveness is mixed.
The Home Office has dismissed these concerns, suggesting the document is now out of date and insists that the Australian model had been successful in reducing illegal journeys and dismantling people smuggling networks.
Charities have warned the “cruel and nasty decision” to “offshore” some asylum seekers more than 4,000 miles away will fail to address the issue, “lead to more human suffering and chaos” while potentially costing millions.
British Red Cross executive director Zoe Abrams said the humanitarian network was “profoundly concerned” about the plans to “send traumatised people halfway round the world to Rwanda”.
“The financial and human cost will be considerable... We are not convinced this drastic measure will deter desperate people from attempting to cross the Channel either.
"People come here for reasons we can all understand, like wanting to be reunited with loved ones, or because they speak the language. Making it harsher may do little to stop them risking their lives,” she said.
In a statement Sophie McCann, Advocacy Officer at Médecins Sans Frontières (MSF) UK called on the Home Office to drop the proposals and "commit to providing safe and legal routes for all people fleeing war and persecution before any more lives are destroyed".
She said one-third of patients the humanitarian organisation worked with had attempted suicide when Australia implemented a similar policy of processing asylum seekers on Nauru island.
"Children as young as nine years-old were trying to kill themselves. This kind of suffering is what awaits refugees in Rwanda if plans go ahead. It is medically and ethically reprehensible."
She added: "Make no mistake, this government knows what the impact of this policy will be – it is knowingly and willingly subjecting refugees to horrific suffering.
"Even at this late stage, we urge the Home Office to reconsider: drop the inhumane policies in the Borders Bill and commit to providing safe and legal routes for all people fleeing war and persecution before any more lives are destroyed.”
SNP Westminster leader Ian Blackford branded the government's plan “evil”.
He told BBC Radio 4’s Today programme: “It’s just chilling, absolutely chilling, to think that people who are coming here for a whole host of reasons – vulnerable people – are going to be taken all the way to Africa to be processed.
“This is not the mark of a civilised society. It’s evil.
“It just turns my stomach to see that our government acting in our name can behave in such a way, and I think a lot of people are going to be quite aghast.”