The government's Rwanda asylum policy, which it says is needed to tackle small boats, is in disarray, after the UK's highest court ruled it is unlawful.

The Supreme Court upheld a Court of Appeal ruling, which said the policy leaves people sent to Rwanda open to human rights breaches.

It means the policy cannot be implemented in its current form.

Rishi Sunak said the government would work on a new treaty with Rwanda and said he was prepared to change UK laws.

The controversial plan to fly asylum seekers to Rwanda and ban them from returning to the UK has been subject to legal challenges since it was first announced by Boris Johnson in April 2022.

The government has already spent £140m on the scheme but flights were prevented from taking off in June last year after the Court of Appeal ruled the approach was unlawful due to a lack of human rights safeguards.

Now that the UK's most senior court has agreed, the policy's chances of being realised without major revisions are effectively ended.

But Mr Sunak told MPs at Prime Minister's Questions that he was ready to finalise a formal treaty with Rwanda and would be "prepared to revisit our domestic legal frameworks" in a bid to revive the plan.

A treaty - which Downing Street has said it will publish in the "coming days" - would upgrade the agreement between the UK and Rwanda from its current status as a "memorandum of understanding", which the government believes would put the arrangement on a stronger legal footing.

The new text would provide the necessary reassurances the Supreme Court has asked for, the prime minister's official spokesman said.

Ministers have been forced to reconsider their flagship immigration policy after 10 claimants argued that ministers had ignored clear evidence that Rwanda's asylum system was unfair and arbitrary.

The legal case against the policy hinges on the principle of "non-refoulement" - that a person seeking asylum should not be returned to their country of origin if doing so would put them at risk of harm - which is established under both UK and international human rights law.

In a unanimous decision, the court's five justices agreed with the Court of Appeal that there had not been a proper assessment of whether Rwanda was safe.

The judgement does not ban sending migrants to another country, but it leaves the Rwanda scheme in tatters - and it is not clear which other nations are prepared to do a similar deal with the UK.

The Supreme Court justices said there were "substantial grounds" to believe people deported to Rwanda could then be sent, by the Rwandan government, to places where they would be unsafe.

It said the Rwandan government had entered into the agreement in "good faith" but the evidence cast doubt on its "practical ability to fulfil its assurances, at least in the short term", to fix "deficiencies" in its asylum system and see through "the scale of the changes in procedure, understanding and culture which are required".

A spokesman for the Rwandan government said the policy's legality was "ultimately a decision for the UK's judicial system", but added "we do take issue with the ruling that Rwanda is not a safe third country".

It leaves Mr Sunak - who has made tackling illegal immigration a central focus of his government - looking for a way to salvage the policy.

In a statement issued after the ruling, the prime minister said the government had been "planning for all eventualities and we remain completely committed to stopping the boats".

He continued: "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. This confirms the government's clear view from the outset."

Mr Sunak is expected to hold a televised press conference in Downing Street at 16:45 GMT on Wednesday.

The Supreme Court decision comes amid the political fallout from the sacking of Suella Braverman on Monday, who, as home secretary had championed the Rwanda policy.

Following the ruling, Mrs Braverman called on Mr Sunak to introduce "emergency legislation" which would block routes of legal challenge, including the European Convention on Human Rights (ECHR) - which some on the Tory right have called for the UK to withdraw from.

Newly appointed Home Secretary James Cleverly told the Commons on Wednesday the government had been "working on a plan to provide the certainty that the court demands" for "the last few months".

He said upgrading the agreement to a treaty "will make it absolutely clear to our courts and to Strasbourg that the risks laid out by the court today have been responded to, will be consistent with international law".

Lee Anderson MP, the deputy chairman of the Conservative Party, urged the government to ignore the Supreme Court and "put planes in the air" anyway.

Natalie Elphicke, Conservative MP for Dover, the landing point for many of the small boats, said the Rwanda policy is "at an end" and "we now need to move forward".

"With winter coming the timing of this decision couldn't be worse. Be in no doubt, this will embolden the people smugglers and put more lives at risk," she continued.

But charity Asylum Aid said the government must "abandon the idea of forcibly removing people seeking asylum to third countries", describing the policy as "cruel and ineffective".

More than 100,000 people have arrived in the UK via illegal crossings since 2018, though the number appears to be falling this year.

In 2022, 45,000 people reached the UK in small boats. The total is on course to be lower for 2023, with the total for the year so far below 28,000 as of November 12.