Brexit?

Helpfully Brexit is a multi-faceted movement, so you can't pin down people as supporting or opposing any particular policy. But it seems a large part of it was driven by opposition to "eastern europeans" - who are in the UK legally under freedom of movement.

If Brexit happens without a new rule being introduced, suddenly millions of people who legally immigrated to the UK will become "illegal". (They can try the naturalisation process, which is long, expensive, not at all guaranteed to succeed, and closed to many categories of people)