Farage deportation plan relies on these conditions - legal expert explains if it could work

Explaining how they plan to tackle what they described as illegal migration, Nigel Farage and his Reform UK colleague Zia Yusuf were happy to disclose some of the finer details - how much money migrants would be offered to leave and what punishments they would receive if they returned.

But the bigger picture was less clear.

How would Reform win a Commons majority, at least another 320 seats, in four years' time - or sooner if, as Mr Farage implied, Labour was forced to call an early election?

How would his party win an election at all if, as its leader suggested, other parties began to adopt his policies?

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Highly detailed legislation would be needed - what Mr Farage calls his Illegal Migration (Mass Deportation) Bill.

But Reform would not have a majority in the House of Lords and, given the responsibilities of the upper house to scrutinise legislation in detail, it could take a year or more from the date of an election for his bill to become law.

Reform's four-page policy document says the legislation would have to disapply:

• The United Nations refugee convention of 1951, extended in 1967, which says people who have a well-founded fear of persecution must not be sent back to a country where they face serious threats to their life or freedom

• The United Nations convention against torture, whose signatories agree not expel, return or extradite anyone to a country where there are substantial grounds to believe the returned person would be in danger of being tortured

• The Council of Europe anti-trafficking convention, which requires states to provide assistance for victims

According to the policy document, derogation from these treaties is "justified under the Vienna Convention doctrine of state necessity".

That's odd, because there's no mention of necessity in the Vienna Convention on the law of treaties - and because member states can already "denounce" (leave) the three treaties by giving notice.

It would take up to a year - but so would the legislation. Only six months' notice would be needed to leave the European Convention on Human Rights, another of Reform's objectives.

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Mr Farage acknowledged that other European states were having to cope with an influx of migrants. Why weren't those countries trying to give up their international obligations?

His answer was to blame UK judges for applying the law. Once his legislation had been passed, Mr Farage promised, there would be nothing the courts could do to stop people being deported to countries that would take them. His British Bill of Rights would make that clear.

Courts will certainly give effect to the will of parliament as expressed in legislation. But the meaning of that legislation is for the judiciary to decide. Did parliament really intend to send migrants back to countries where they are likely to face torture or death, the judges may be asking themselves in the years to come.

They will answer questions such as that by examining the common law that Mr Farage so much admires - the wisdom expressed in past decisions that have not been superseded by legislation. He cannot be confident that the courts will see the problem in quite the same way that he does.