As things stand, the UK government’s plan to send some asylum seekers to Rwanda is costing at least £290m – and it is still nowhere near take-off.

Rishi Sunak’s proposal to make the currently unlawful scheme a reality is shaping up to be politically and legally explosive with potential fights in both Parliament and the courts.

What happened last year?

In last November’s Supreme Court ruling, five of its justices unanimously ruled that Rwanda was not a safe country to send asylum seekers. The court upheld a previous ruling, in the Court of Appeal, which said the policy would leave people sent to Rwanda open to human rights breaches.

Their judgement said the country’s asylum system was deeply flawed and the UK government had not followed existing laws in how to assess the risks to anyone sent there.

Those laws were a combination of safeguards in the European Convention on Human Rights – which MPs long ago voted into British law – and other measures passed by MPs to make sure that ministers cannot mistreat people who have fled for their lives.

That has lead to the government putting forward a new plan – formally the Safety of Rwanda Bill.

The key element of the government’s package aims to deal with the Supreme Court defeat. Parliament will be asked to declare that Rwanda is “conclusively” safe and, simultaneously, ban British judges from ever saying it is not.

Ministers say that a parallel treaty with Rwanda deals with the concerns about the state of the Rwandan asylum system because the country has made promises to improve matters and the UK will pay for extra monitoring and training.

Supporters of the Bill say there is nothing wrong in this. Parliament would be democratically voting in a new law to help deliver one of the government’s priorities – as is its sovereign right.

Critics say it’s a constitutional smash and grab, tying the hands of independent judges who have the duty to consider evidence and facts.

In an act of belt and braces, the Bill also orders British judges to ignore sections of the Human Rights Act that set out how they should interpret minimum safeguards for fair and just treatment – including the right not to be tortured and the right to a fair hearing. It also prevents judges from considering other international safeguards.

Finally, it says our courts must ignore any other British law that stands in the way of finding the country to be safe.

Critics say that the government has come up with a Bill that allows it to pick and choose when to follow laws and rules while simultaneously expecting Rwanda to adhere to international standards at all times.

What happens if the bill is passed unamended?

Tory backbenchers on the right of the party have already backed plans to make amendments to the Bill.

But if the PM sees off the challenge from the Tory right – and more on those MPs in a moment – it could lead to an unprecedented constitutional stand-off between Parliament and judges.

The Supreme Court cannot strike down primary legislation – but it has the power to make a “declaration of incompatibility”.

This is a rare judgment that says an Act of Parliament should be rethought because it is totally at odds with the European Convention of Human Rights safeguards that are baked into British law.

If the Supreme Court makes such a declaration, the law in question will essentially be in legal limbo. History shows that the government essentially respects the court’s view and sets about trying to fix the problem.

But if ministers pressed ahead with flights to Rwanda, it is a racing certainty that claimants would exercise their legal right to go the European Court of Human Rights to say they have been denied a legal remedy by the UK.

The court in Strasbourg would then have to consider whether to temporarily block a flight until the case has been fully considered – potentially walking into a political bear pit on the eve of a General Election.

So where does this leave the plan?

Government lawyers have warned ministers the Bill is more likely than not to fall apart under sustained legal challenge. And that’s why Tory rebels say it needs to go further.

They want Parliament to instruct ministers to treat any Strasbourg injunction as legally irrelevant.

If this amendment passed, some ministers would be in a tight spot. The Attorney General Victoria Prentis and the Lord Chancellor Alex Chalk have a duty to uphold the law – including international obligations.

The rebels argue that their amendments don’t break international law – and the plan can only become practicably workable with a “broader block” on human rights law to prevent appeals other than in the most exceptional of cases, such as if someone could not fly due to pregnancy.

They also want to strip the Supreme Court of its power to make a declaration of incompatibility in relation to Rwanda.

The practical effect of that proposal would be to take away one of the Supreme Court’s constitutional duties – a political cannon ball fired across Parliament Square and through its windows.

What happens if MPs back the plan?

If MPs vote in favour of the Bill, it will then be sent to the House of Lords for debate and consideration.

But there is no certainty the House of Lords will support it. Peers could hold it up because it was not a Conservative manifesto commitment at the last election. Ministers can’t overrule the Lords because there is not enough Parliamentary time left to use those powers.

But just supposing the Lords backed down and the plan became law, there is no guarantee we will see a flight take off.