LONDON — The British government said Friday that it plans to start putting asylum-seekers on one-way flights to Rwanda within weeks, as it defended a deal that has outraged refugee groups and humanitarian organizations.

Britain and Rwanda announced Thursday that they had struck an agreement that will see some people arriving in the U.K. as stowaways on trucks or in small boats sent 4,000 miles (6,400 kilometers) to the East African country, where their asylum claims will be processed and, if successful, they will stay.

The British government says the plan will discourage people from making dangerous attempts to cross the English Channel, and put people-smuggling gangs out of business.

But critics of the Conservative government said legal and political hurdles mean the flights may never happen. They accused Prime Minister Boris Johnson of using the headline-grabbing policy to distract attention from his political troubles. Johnson is resisting calls to resign after being fined by police this week for attending a party in his office in 2020 that broke coronavirus lockdown rules.

Conservative lawmaker Andrew Griffith, a senior Johnson adviser, said the flights to Rwanda could start “in weeks or a small number of months.”

Migration Minister Tom Pursglove said the drastic plan was needed to deter people trying to reach Britain in dinghies and other boats from northern France. More than 28,000 migrants entered the U.K. across the Channel last year, up from 8,500 in 2020. Dozens have died, including 27 people in November when a single boat capsized.

“Nobody should be coming in a small boat to come to the United Kingdom,” Pursglove told Sky News. “We quite rightly have a rich and proud history in this country of providing sanctuary for thousands of people over the years. …. But what we can’t have, and we can’t accept, is people putting their lives in the hands of these evil criminal gangs, and that’s why we think it is important that we take these steps.”

The deal — for which the U.K. has paid Rwanda 120 million pounds ($158 million) upfront — leaves many questions unanswered, including its final cost and how participants will be chosen. The U.K. says children, and families with children, will not be sent to Rwanda.

Refugee and human rights groups called the plan inhumane, unworkable and a waste of taxpayers’ money. The United Nations’ Refugee Agency urged Britain and Rwanda to reconsider.

“Such arrangements simply shift asylum responsibilities, evade international obligations, and are contrary to the letter and spirit of the Refugee Convention,” said the agency’s Assistant High Commissioner for Protection, Gillian Triggs. “People fleeing war, conflict and persecution deserve compassion and empathy. They should not be traded like commodities and transferred abroad for processing.”

Previous schemes to “offshore” asylum-seekers have been highly controversial.

In 2013, Australia began sending asylum-seekers attempting to reach the country by boat to Papua New Guinea and the tiny atoll of Nauru, vowing that none would be allowed to settle in Australia. The policy all but ended the people-smuggling ocean route from Southeast Asia, but was widely criticized as a cruel abrogation of Australia’s international obligations.

Critics of the U.K.-Rwanda plan say it is certain to face legal challenges. The prime minister acknowledged Thursday it would likely be challenged in court by what he called “politically motivated lawyers” out to “frustrate the government.”

The Law Society of England and Wales, which represents solicitors, chastised the government for offering “misleading suggestions that legal challenges are politically motivated.”

“Legal challenges establish if the government is abiding by its own laws,” said society President I. Stephanie Boyce. “If the government wishes to avoid losing court cases, it should act within the law of the land.”