Britain’s Supreme Court ruled unanimously on Wednesday that the government’s plan to send illegal migrants who arrived on UK shores by crossing over from France in small boats to Rwanda, an African country with a questionable human rights record, was illegal.

The decision is a devastating blow for the Prime Minister Rishi Sunak, who had made stopping the boats a central plank of his government’s policies.

He is already facing a backlash from right wing Tories after his sacking of the home secretary Suella Braverman.

She had come up with the Rwandan plan and in a blistering letter on Tuesday accused Rishi of lack of leadership and having no effective plan to stop the boats. A hardliner on immigration, she had especially wanted to slash the number of students and their dependent relatives coming from India.

The real – and much more important piece of good news on Wednesday – was drowned out by the Supreme Court ruling. Rishi had pledged to halve inflation to 5.4 per cent by the end of 2023. According to the Office of National Statistics, it dropped from 6.7 per cent in August to 4.6 per cent in October.

The Supreme Court backed a ruling from the Appeal Court which had overturned the decision made by the High Court that the government’s Rwandan plan was legal.

Last year, 45,756 people crossed the channel in small boats from France, with Iran, Albania, Iraq and Afghanistan providing the greatest numbers. In the first six months of 2023, the figure has dropped to 11,500 – representing a decline of 10 per cent.

One difference between Suella and Rishi is that she was willing for Britain to quit the European Court of Human Rights if it opposed the Rwandan plan. The Prime Minister was reluctant to do so.

Lord Reed explained his judgement: “The legal test which has to be applied in this case is whether there are substantial grounds for believing that asylum seekers sent to Rwanda would be at real risk of refoulement (this means sending people back to their home countries).

“In the light of the evidence which I have summaries, the Court of Appeal concluded that there were such grounds.

“The home secretary’s appeal is therefore dismissed.”

The government has invested heavily in the Rwandan plan which has now effectively been blown out of the water by the Supreme Court. But Rishi’s spokesman said in a statement: “We have seen today’s judgment and will now consider next steps. 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.”

The ruling was welcomed by Katy Chakrabortty, head of policy and advocacy at the charity Oxfam. She said it was a “great relief for many”, calling the Rwandan plan an inhumane scheme. “It sought to punish rather than protect those fleeing conflict and persecution.”