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.

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.

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

But the heads of the UN agencies for refugees and human rights 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."

The Council of Europe also called for the new law to be scrapped and said it raised "major issues about the human rights of asylum seekers and the rule of law more generally." (AFP)