In a very welcome decision the Supreme Court has found that the UK Government's intention to send asylum seekers to Rwanda (the Rwanda plan) is unlawful. Under the plan, asylum seekers entering the UK unlawfully would be sent to Rwanda where their claims would be assessed. If successful, they would remain in Rwanda as refugees – they would not return to the UK.

In a unanimous decision, 5 of the UK's most senior judges found this plan to be unlawful. This isn't hugely surprising to those who have been following along but many asylum seekers and campaigners will be breathing a sigh of relief. Rwanda is improving its legal infrastructure and human rights record but these were, the Supreme Court decided, still far below acceptable limits.

The key term through the proceedings was the word "refoulement" – pronounced with your best French accent. Refoulement is, essentially, sending someone entitled to refugee status either to their own country, or to another country which in turn is likely to send them back home. Refoulement can be direct or indirect. Therefore, if an asylum seeker is not harmed in Rwanda but is required to leave Rwanda for their home country where they are harmed, the UK has still refouled that asylum seeker. The UK cannot wash its hands of that individual after removing them to Rwanda. The UK must ensure their safety.

This meant that, even if asylum seekers wouldn't be directly harmed in Rwanda, if they were at risk of being sent to their home countries, or even neighbouring countries, where they would be harmed, then the Rwanda Plan was unlawful.

The Supreme Court reviewed evidence from the UNHCR – the UN's expert body on protecting asylum seekers and refugees around the world. The High Court, hearing an earlier case on this matter, had minimised the weight that could be attached to this report for various reasons. The Supreme Court felt differently and gave the UNHCR's report much greater significance in their consideration acknowledging the UNHCR as an expert body best placed to comment on the plan.

The UNHCR report is not pretty reading for supporters of the Rwanda plan and you can probably guess what it says.

The Supreme Court also referred to a very similar plan Rwanda made with Israel several years ago. Under that plan, asylum seekers in Israel were sent to Rwanda but many were then returned to their home countries or sent to neighbouring countries where they were harmed. Needless to say, the Supreme Court thought Rwanda's refoulement of asylum seekers under their agreement with Israel strongly suggested that refoulement was a real risk to those sent by the UK.

It's important to note that it is the conditions in Rwanda that render the plan unlawful. The idea of using a third country to process and decide asylum claims is not inherently unlawful if those asylum seekers are protected throughout the proceedings. Should conditions in Rwanda change, or should the UK make an agreement with another country, then the plan may be resurrected.

No one sensible thinks the Rwanda plan is a good idea but the current Government, for various reasons, has made it a flagship policy despite recent changes in personnel. Prime Minster Rishi Sunak has indicated that they may legislate to prevent future legal challenges, something former immigration minister Damien Green described today as "unconservative". Seeking to avoid the scrutiny of the courts in such a way is a trick pulled by Russian, Belarus and the right-wing Polish government in recent times; it should trouble everyone who supports the rule of law in a democratic country.

Parliament is sovereign in the UK so, assuming the Government decides to spend political capital in this way, it's a legal possibility. However, this would be such a complicated and politically fraught task that it seems highly unlikely.

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