A law that will enable Britain to send some asylum-seekers on a one-way trip to Rwanda suffered a setback on Wednesday after Parliament’s upper chamber pressed its attempt to amend the contentious legislation.

The House of Lords inserted amendments into the Safety of Rwanda Bill, sending it back to the lower House of Commons in a process known as parliamentary ping-pong.

The government had hoped members of the Lords would stop blocking the bill on Wednesday, relenting to the parliamentary rule that the unelected Lords ultimately can’t overrule the elected Commons. The Lords’ resistance underlines the strength of opposition in the upper house, where the governing Conservative Party does not have a majority.

The bill is still overwhelmingly likely to become law, but the latest move delays its passage, likely until next week.

The legislation will pave the way for deportation flights to take off – though opponents plan new legal challenges that could keep them grounded.

The Rwanda plan is key to Prime Minister Rishi Sunak ’s pledge to “stop the boats” bringing unauthorized migrants to the U.K. across the English Channel, and Sunak has repeatedly said the first flights will take off in the spring.

Home Office Minister Michael Tomlinson told lawmakers on Wednesday that the law is needed because “we simply cannot stand by and allow people smugglers to control who enters our country and to see more lives being lost at sea.”

“We have an obligation to the public and to those who are being exploited by criminal gangs to stop this vile trade and to protect our borders,” he said.

It has already been two years since Britain and Rwanda signed a deal that would see migrants who cross the English Channel in small boats sent to the East African country, where they would remain permanently. The plan has been challenged in the courts, and no one has yet been sent to Rwanda under an agreement that has cost the U.K. at least 370 million pounds ($470 million).

Sunak’s government says the plan will deter people from making dangerous journeys across one of the world’s busiest shipping lanes and break the business model of people-smuggling gangs. Human rights groups and other critics say it is unworkable and unethical to send migrants to a country 4,000 miles (6,400 miles) away that they don’t want to live in.

The Safety of Rwanda Bill is designed to overcome a ban on sending migrants to Rwanda imposed by the U.K. Supreme Court, which ruled in November that the East African country is not a safe destination for asylum-seekers because there is a risk they could be returned to the conflict-wracked home countries they’d fled.

In response, Britain and Rwanda signed a treaty pledging to strengthen protections for migrants. Sunak’s government argues the treaty allows it to pass the new law, which pronounces the country safe, making it harder for migrants to challenge deportation and allows the British government to ignore injunctions from the European Court of Human Rights that forbid removals.

Human rights groups, refugee charities, senior Church of England clerics and many legal experts have criticized the legislation. In February a parliamentary rights watchdog said the Rwanda plan is “ fundamentally incompatible ” with the U.K.’s human rights obligations.

The Safety of Rwanda Bill was approved in January by the House of Commons, where Sunak’s Conservatives have a majority, but met strong opposition in Parliament’s upper chamber, the House of Lords. Members of the Lords repeatedly inserted amendments to water down the legislation, including by exempting Afghans who worked with British forces from deportation.

The Commons rejected them all, but the Lords has repeatedly restored the changes.

The government has refused to accept any amendments. A spokesman for Prime Minister Rishi Sunak said Wednesday: “We’re not considering concessions.”

Britain’s main opposition parties oppose the legislation. Scottish National Party lawmaker Alison Thewlis urged the Labour Party to repeal the law if it wins an election later this year, as polls suggest it will.

“The Rwanda Bill is a turd which cannot be polished,” she said. “It is absolutely disgusting and objectionable in every sense.”