Story highlights House of Lords votes in favor of amendment calling for UK to protect EU citizens' rights

Defeat in Lords sets off parliamentary 'ping-pong'

London (CNN) British Prime Minister Theresa May suffered her first Brexit bill defeat Wednesday, over the rights of EU citizens living in the UK.

Members of the House of Lords, the upper house of the UK parliament, voted in favor of an amendment that called for EU nationals legally resident in Britain to be guaranteed the same status after the UK leaves the European Union.

The measure was passed by 358 votes to 256 in the Lords , where May's Conservative Party does not command a majority.

The amended bill must now be sent back for consideration to the House of Commons, potentially threatening Downing Street's March 31 deadline to trigger Article 50, the formal start to the Brexit process. In the Commons in February, MPs passed the bill unamended after being assured that the rights of EU nationals would be protected.

Jeremy Corbyn, leader of the opposition Labour Party, reacted to the news on Twitter, writing, "The government must now do the decent thing and guarantee the rights of EU citizens living in the UK."

Great news. The government must now do the decent thing and guarantee the rights of EU citizens living in the UK https://t.co/HWJUfxfqRl — Jeremy Corbyn MP (@jeremycorbyn) March 1, 2017

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