Migrants picked up at sea attempting to cross the English Channel from France, disembark from Border Force vessel 'Defender' after arriving at the Marina in Dover, southeast England, on January 17, 2024. [AFP]

The UK readied on Tuesday to start detaining migrants within days for deportation to Rwanda after the controversial plan got parliament's approval, sparking outrage from the UN and rights groups.

The new legislation -- a flagship policy of the Conservative government that aims to curb irregular cross-Channel migration from northern France -- cleared its final hurdle after a marathon late-night parliamentary tussle on Monday.

Just hours later, French police said at least five migrants, including a child, died overnight during an attempt to cross the busy shipping lane in a small boat, underlining the perilous nature of the crossings.

Under the UK scheme, undocumented asylum seekers arriving in Britain would be sent to Rwanda, where their asylum claims would be examined and, if approved, would allow them to stay in Rwanda.

Prime Minister Rishi Sunak says deportations are expected to begin within 10-12 weeks, with migrants identified for the first flight due to be detained and held from as early as this week.

In Kigali, the government said it was "pleased" to see the bill passed and was looking forward to "welcoming those relocated to Rwanda".

The east African country is reportedly due another £50 million ($62 million) payment from London when the legislation formally becomes law in the coming days, as part of the deal that has already cost UK taxpayers hundreds of millions of pounds (dollars).

But the heads of the UN agencies for refugees and human rights as well as other NGOs and UK faith leaders all decried the plan.

Atuib, a Sudanese national, arrived on a small boat last year and has been housed on the Bibi Stockholm, a government-chartered accommodation barge moored off the southern English coast.

"If I am sent to Rwanda, I will be sent to Sudan," Atuib, 23, told AFP. "Rwanda is not good and not safe. It's safe here and there's no war."

Another migrant on the barge, Martin, 28, from South Africa, added: "It's better to kill me than take me to Rwanda."

'Deterrent’

The UN agencies warned it threatened the rule of law and set "a perilous precedent globally".

They urged the UK to instead "take practical measures to address irregular flows of refugees and migrants, based on international cooperation and respect for international human rights law".

Sunak said the "landmark legislation" would deter record numbers of migrants crossing the Channel from northern France and disrupt people-smuggling gangs behind them.

"Our focus is to now get flights off the ground, and I am clear that nothing will stand in our way of doing that and saving lives," he said.

The legislation, which will compel judges to regard Rwanda as a safe third country, now goes to head of state King Charles III for royal assent. He has reportedly criticised the plan as "appalling".

It also gives decision-makers on asylum applications the power to disregard sections of international and domestic human rights law to get around a UK Supreme Court ruling that said sending migrants on a one-way ticket to Kigali was illegal.

The government endured a months-long parliamentary battle to pass it, with the unelected House of Lords chamber, which scrutinises bills, repeatedly sending the legislation back to the House of Commons with amendments.

Peers, as Lords members are known, eventually conceded defeat late Monday.

Costly

Amnesty UK chief executive Sacha Deshmukh criticised the law for risking the UK's reputation for "independence of the judiciary" and said it would be "welcomed by authoritarians who want to copy".

Sunak's government has been under mounting pressure to cut record "small boat" arrivals, particularly after a promise of a tougher approach to immigration after the UK left the European Union.

The Rwanda scheme was first proposed in 2022 but has been beset by legal challenges ever since and two years on, no migrants have been deported.

The National Audit Office, a public spending watchdog, has estimated it will cost the UK £540 million ($665 million) to deport the first 300 migrants -- nearly £2 million per person.

Critics say the scheme is unworkable and, given the small numbers involved, would do little to cut the backlog of asylum claims.

Rwanda -- a tiny nation of 13 million people -- lays claim to being one of the most stable countries in Africa. But rights groups accuse veteran President Paul Kagame of ruling in a climate of fear, stifling dissent and free speech.

Sunak's plans could still be held up by legal challenges, while UN rights experts have suggested that airlines and aviation regulators could fall foul of internationally protected human rights laws if they take part in deportations.