Britain's unelected House of Lords on Wednesday handed the government a stinging — though likely temporary — defeat on its plans to leave the European Union, resolving that EU citizens should be promised the right to stay in the U.K. after it quits the bloc. By a vote of 358 to 256, Parliament's upper chamber inserted a clause protecting EU nationals' status into a bill authorizing the government to begin EU exit talks.

The change means the bill must return to the lower House of Commons for deliberation, delaying final approval just weeks ahead of May's deadline for starting Brexit negotiations by the end of this month. Before the vote, May said her timetable for triggering Article 50 of the European Union's Lisbon treaty, which starts a two-year negotiating period, would not change.

The government said it was "disappointed" by the Lords' vote, and is expected to try to overturn the amendment in the Commons later this month.

By leaving the EU, Britain will be withdrawing from the bloc's policy of free movement, which allows citizens of the bloc's 28 member states to live and work in any of the others. That leaves 3 million EU nationals in the U.K., as well as 1 million Britons in other member countries, uncertain whether they will be able to stay in their jobs and homes once Britain reasserts control over EU immigration.

The government has said repeatedly that it plans to guarantee the right of EU citizens to remain in Britain as long as U.K. nationals living elsewhere in the bloc get the same right. Critics accuse the government of treating people as leverage in the divorce negotiations.

The Lords' amendment commits the government to guaranteeing within three months that EU citizens living legally in Britain when the bill is passed "continue to be treated in the same way with regards to their EU-derived rights."