Controversial UK government plans for deporting asylum seekers to Rwanda cleared their final hurdle on Monday, after a marathon tussle between the upper and lower chambers of parliament lasting late into the night.

Prime Minister Rishi Sunak and his ruling Conservatives have been seeking to push through legislation that will compel judges to regard the east African nation as a safe third country.

They also want to give 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 sending migrants on a one-way ticket to Kigali was illegal.

But the government faced a parliamentary battle to do so, with the upper chamber House of Lords, which scrutinises bills, repeatedly sending the proposed legislation back to the lower House of Commons with amendments.

Peers, who have criticised the bill as inadequate, notably wanted a requirement that Rwanda could not be treated as safe until an independent monitoring body said so.

They also wanted an exemption for agents, allies and employees of the UK overseas, including Afghans who fought alongside British armed forces, from being removed.

MPs in the Commons, where the Tories have a majority, voted down every amendment and asked the Lords to think again in a back-and-forth process known as "parliamentary ping pong".

The unelected upper chamber, where there is no overall majority for any party, dug in their heels.

But shortly before midnight (2300 GMT) they eventually conceded to the will of elected MPs and agreed to make no further amendments, ending the deadlock and ensuring the bill will now receive royal assent to pass into law.

Sunak's government has been under mounting pressure to cut record numbers of asylum seekers crossing the Channel from northern France in small boats, particularly following a promise of a tougher approach to immigration after the UK left the European Union.

- Challenges -

The Rwanda scheme -- criticised by UN human rights experts and groups supporting asylum seekers -- has been beset by legal challenges since it was first proposed in 2022.

That year, the first deportees were pulled off a flight at the last minute after an injunction from the European Court of Human Rights. Two years on, no migrants have been sent.

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.

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

Other critics say it sets a dangerous precedent of parliament legislating on an issue already deemed illegal by the courts, and will damage the UK's international standing and moral authority.

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 announced earlier on Monday that the government was ready and had plans in place for the first flights to take off in 10 to 12 weeks, promising a wave of deportations "come what may" over the summer months.

The prime minister is banking on the flagship "stop the boats" policy to act as a deterrent and give his beleaguered Tory party an electoral boost as the country prepares to go to the polls later this year.

The Conservatives have consistently trailed the main opposition Labour party in opinion polls and are on course to be dumped out of power after 14 years.

Sunak's plans could still be held up by legal challenges, and 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.