A bench of five in the Supreme Court unanimously upheld a ruling from the Court of Appeal that there was a real risk of deported refugees having their claims in Rwanda wrongly assessed or being returned to their home countries where they could face persecution.

The government had said that the £140 million Rwanda scheme would deter growing numbers of asylum seekers reaching the UK via small boats coming across the Channel.

President of the court, Lord Reed, said the justices agreed unanimously with the lower court that there was a real risk of claims being wrongly determined in Rwanda.

He cited evidence from the United Nations’ refugee agency, the UNHCR, highlighting the failure of a similar scheme between Israel and Rwanda.

After judgment was handed down this morning, prime minister Rishi Sunak said: “This was not the outcome we wanted, but we have spent the last few months planning for all eventualities and we remain completely committed to stopping the boats.

“Crucially, the Supreme Court — like the Court of Appeal and the High Court before it — has confirmed that the principle of sending illegal migrants to a safe third country for processing is lawful.”

Home secretary James Cleverly said: “Our partnership with Rwanda, while bold and ambitious, is just one part of a vehicle of measures to stop the boats and tackle illegal migration.

“But clearly there is an appetite for this concept. Across Europe, illegal migration is increasing and governments are following our lead: Italy, Germany and Austria are all exploring models similar to our partnership with Rwanda.”

Sonya Sceats, chief executive of Freedom from Torture, said: “This is a victory for reason and compassion. We are delighted that the supreme court has affirmed what caring people already knew: the UK government’s ‘cash for humans’ deal with Rwanda is not only deeply immoral, but it also flies in the face of the laws of this country.

“The stakes of this case could not have been higher. Every day in our therapy rooms we see the terror that this scheme has inflicted on survivors of torture who have come to the UK seeking sanctuary.”