Millions of EU citizens living in the UK and Britons settled in Europe face an uncertain future after Theresa May warned that their status would be part of the Brexit negotiations. Vote Leave said during the referendum campaign that EU citizens already living and working in the UK would retain their rights under the 1969 Vienna convention which protects rights acquired before withdrawal from treaties. Some lawyers have queried this assertion. “There is no cast-iron guarantee on acquired rights in the event Britain leaves the EU,” said Professor Sionaidh Douglas-Scott, Anniversary Chair in Law and Co-Director at the Centre for Law and Society in a Global Context (CLSGC) at Queen Mary University of London (QMUL). “If you leave the EU you are no longer a member of the club that gives you those rights.”