LONDON (AP) — The upper house of Britain’s Parliament has urged the Conservative government not to ratify a migration treaty with Rwanda. It’s a largely symbolic move, but signals more opposition to come for the stalled and contentious plan to send some asylum-seekers on a one-way trip to the African nation.

The House of Lords voted by 214 to 171 on Monday evening to delay the treaty that paves the way for the deportation plan. The treaty and an accompanying bill are the pillars of Prime Minister Rishi Sunak ’s bid to overcome a block on the deportations by the U.K. Supreme Court.

Members of the Lords, who are appointed rather than elected, backed a motion saying Parliament should not ratify the pact until ministers can show Rwanda is safe.

John Kerr, a former diplomat who sits in the Lords, said the Rwanda plan was “incompatible with our responsibilities” under international human rights law.

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“The considerations of international law and national reputation ... convince me that it wouldn’t be right to ratify this treaty at any time,” he said.

The vote has little practical impact, because the House of Lords can’t block an international treaty, and the government says it will not delay. However, ignoring the demand could later be used against the government in a legal challenge.

Lawmakers in the House of Commons approved the bill last week, but only after 60 members of Sunak’s governing Conservatives rebelled in an effort to make the legislation tougher.

Monday’s vote indicates the strength of opposition in the House of Lords. Many there want to water down the bill — and, unlike in the Commons, the governing Conservatives do not have a majority of seats.

The Lords will begin debating the bill next week. Ultimately the upper house can delay and amend legislation but can’t overrule the elected Commons.

The Rwanda policy is key to Sunak’s pledge to “stop the boats” bringing unauthorized migrants to the U.K. across the English Channel from France. Sunak argues that deporting unauthorized asylum-seekers will deter people from making risky journeys across the English Channel and break the business model of people-smuggling gangs.

London and Kigali made a deal almost two years ago under which migrants who reach Britain across the Channel would be sent to Rwanda, where they would stay permanently. Britain has paid Rwanda at least 240 million pounds ($305 million) under the agreement, but no one has yet been sent to the East African country.

Human rights groups have criticized the plan as inhumane and unworkable. After it was challenged in British courts, the U.K. Supreme Court ruled in November that the policy was illegal because Rwanda isn’t a safe country for refugees.

In response to the court ruling, Britain and Rwanda signed a treaty pledging to strengthen protections for migrants. Sunak’s government argues the treaty allows it to pass a law declaring Rwanda a safe destination.

If approved by Parliament, the law would allow the government to “disapply” sections of U.K. human rights law when it comes to Rwanda-related asylum claims and make it harder to challenge the deportations in court.