Some British proposals for EU reform are "highly problematic", a EU Commission spokesperson has said.
Asked at a news conference for the Commission's initial reaction to the letter, chief spokesman Margaritis Schinas said:
Prima facie we see a number of elements which appear to be feasible, like finding ways to increase the role of national parliaments, some issues which are difficult, like ever closer union and relations between the euro ins and outs, and some things which are highly problematic as they touch upon the fundamental freedoms of the internal market. Direct discrimination between EU citizens clearly falls into this last category.
The 30-year-old system that has protected free movement across the continent is under severe threat because of the mass influx of refugees.
We got the detail of David Cameron's demands today - but it would be hard to argue we got anything new.
Read David Cameron's letter to President of the European Council Donald Tusk in full here.