The House of Commons has passed the amendment to the UK Withdrawal Agreement that seeks to ringfence rights for Britons living in the EU and EU citizens living in the UK, regardless of the circumstances under which the exit eventually occurs.
The amendment was tabled on Wednesday morning by MP for South Leicestershire, Alberto Costa, whose resignation from his position as Parliamentary Private Secretary that same morning caused confusion.
While early reports suggested Costa had been sacked, it later emerged that a parliament convention expects any MP with a parliamentary title to resign from their post in order to table an amendment to any bill.
Speaking in the House of Commons after his amendment had been passed, the anti-Brexit MP, who is the son of Italian immigrants, said that citizens "should never have been used as a bargaining chip" and that "ringfencing" their rights "should have been done at the outset" of Brexit negotiations.
His amendment, which calls for the part of the withdrawal agreement that includes citizens' rights to be implemented at "the earliest opportunity", regardless of the outcome of negotiations, gained cross-party support.
Costa thanked the umbrella group British in Europe, which represents Britons living in other EU countries, as well as UK-based lobby The Three Million, representing EU citizens living in the UK, for campaigning to protect people's rights. Costa concluded his four-minute speech by saying, "Citizens' rights are not about party politics but about people."
There were calls from politicians across the political spectrum for Costa to be offered back his role as PPS to the Scotland Minister, David Mundell; MP Caroline Lucas described the convention as "an ass" and said that Costa "should be back in his job".
Theresa May will now have to take the amendment back to Brussels, to try to seek agreement on the issue.