
http://blogs.lse.ac.uk/brexit/2018/11/23/hubert-butler-essay-prize-what-happened-to-europe-without-frontiers/

https://www.eurofound.europa.eu/sites/default/files/ef_publication/field_ef_document/ef1609en.pdf

https://www.instituteforgovernment.org.uk/sites/default/files/publications/IfG_views-eu-27-v5_WEB.pdf