The Prime Minister’s promise was a straightforward one – he vowed to “stop the boats” as thousands of asylum seekers crossed the Channel without authorisation.

But as crossings continued, the pledge has been complicated by legal challenges arguing that forcibly removing migrants to Rwanda is against their rights and the law.

The Supreme Court delivered Rishi Sunak a blow by ruling that his policy was unlawful.

Five of the UK’s most senior justices ruled the scheme could not go ahead as it was.

They cited a long list of concerns about Rwanda as they concluded there was a real risk that genuine refugees could be sent back to their country of origin, where they would face “ill-treatment”.

Now Home Secretary James Cleverly has travelled to Kigali to sign a fresh treaty as part of Mr Sunak’s goal of making the plan to send migrants to the African nation legally watertight.

– What does the treaty contain?

Mr Sunak is pinning his hopes on soothing the court’s concerns by agreeing a new legally binding treaty with Rwanda.

Home Office officials say the treaty centres on preventing what is known as “refoulement”, where asylum seekers are removed and returned to a country where they face persecution, to satisfy concerns raised in the Supreme Court’s findings.

The treaty, which needs to be ratified by both the UK and Rwandan parliaments to make it internationally binding, seeks to make sure the country does not remove migrants and send them back to their home country, or another country, after they have arrived from the UK.

As part of the treaty, a new appeals process will be established within Rwanda’s high court to handle exceptional cases, for example if someone living in the country under the scheme committed a crime, if the Government decides it will seek to deport the asylum seeker.

British and Commonwealth judges, as well as Rwandan judges, will preside over the appeal court hearings. Rulings will decide whether an asylum seeker remains in Rwanda or is sent back to the UK, it is understood.

But details of the fresh deal, which officials say is the first of its kind, are scant, with some of the specifics still being thrashed out, so the exact arrangements of how this will work in practice are still unclear.

It is not yet known how many British or Commonwealth judges would be involved and the extent of their jurisdiction as part of the process.

An independent monitoring authority, set up to oversee the policy between the two countries, was initially only to visit the country and review the work once a year, but will now meet more regularly in a bid to bolster the level of scrutiny the scheme is given, according to officials.

Mr Sunak wants to go further to reduce the chances of a court blocking the new treaty by bringing forward “emergency” legislation to ask Parliament to confirm it believes Rwanda is a “safe” country.

Supreme Court president Lord Reed and his four justices ruled there were “substantial” grounds to believe there was a “real risk” of refugees being returned to their home countries.

Elevating the deal from a “memorandum of understanding” to a treaty that has been ratified by Parliament could strengthen it in the court’s minds.

– What problems must the treaty overcome?

The Supreme Court judgment highlighted a series of grave concerns about Rwanda that would need addressing before they could consider the policy lawful.

Refoulement was the chief concern, which the justices said was seen in a “similar” deal Rwanda had with Israel, and was continuing.

Evidence from the UNHCR, the United Nations refugee agency, showed there was 100% rejection for nationals from three war-torn countries between 2020 and 2022.

Home Office figures from the same period showed the UK granted 74% of cases from Afghanistan, 98% from Syria and 40% from Yemen.

The Supreme Court said there was “evidence of a culture within Rwanda of, at best, inadequate understanding of Rwanda’s obligations under the Refugee Convention”.

Evidence was presented that Kigali had a “poor human rights record”, citing British police warning Rwandans in the UK of credible plans by the country’s government to kill them.

Concerns of political and media freedom were also raised, as was the inability of the Rwandan courts to act independently of the government.

The UNHCR suggested there had been more than 100 cases of refoulement after the UK agreed its deal with Rwanda.

– Is this enough to get flights off the ground?

The UK Government believes the changes updating the initial deal should answer reservations set out in the Supreme Court’s ruling last month.

Officials have said the Rwanda plan is a novel one and so the Government is testing what can be done. But they reiterated the primary purpose of the plan in ministers’ eyes, which is to act as a deterrent to asylum seekers in a bid to curb the number of Channel crossings.

– And at what cost?

Not a penny of the £140 million already paid to Kigali can be clawed back, and the new treaty is expected to add to the cost.

Mr Cleverly insisted that Britain was not handing over more money directly to secure the treaty, but acknowledged further payments would be used to cover the fresh burdens on Rwanda.

“Of course, when a country is taking on responsibilities as Rwanda is doing, it is right and proper that there is remuneration to reflect the additional costs that they are bringing on,” he told broadcasters.

But Mr Cleverly declined to say how much.

– Is there a plan C if this does not work?

Facing calls from the Tory right to pull out of the European Convention on Human Rights (ECHR), Mr Sunak offered a glimmer of hope.

He said he was “prepared to change our laws and revisit those international relationships” if they were still “frustrating” his plans.

But Mr Cleverly previously told the Commons he did not believe disregarding the ECHR or the Refugee Convention was necessary.

The Home Secretary made clear that the judgment was not contingent on the ECHR.

Delivering the court’s ruling, Lord Reed made clear it was not just the ECHR that was relevant to the case, but other treaties such as the UN conventions against torture and its covenant on civil rights.

He cited three pieces of domestic legislation as binding the UK on the principle of non-refoulement and said the Human Rights Act gave effect to the ECHR.

– How soon will flights begin?

Even if the simpler treaty route was successful, it could take more than 40 sitting days to be ratified in Parliament. No 10 said the agreement could be laid in the House in the “coming days”.

But further legal challenges could halt flights again.

Navigating the legal minefield of amending multiple domestic laws would take months, at least, and disregarding international commitments could create a row with allies.

Downing Street was unable to say whether flights to Kigali would take off before the next election, which is expected within a year.

Mr Sunak said ministers are “working extremely hard to make sure that we can get a plane off as planned in the spring”.

But he repeatedly refused to commit that the first flight would take off before the next election.