Romania and Bulgaria- the so-called 'A2 countries'- joined the EU on 01 January 2007. However, their rights to work and claim benefits were limited for their first seven years of EU membership.
Up until 01 January 2014, nationals of Bulgaria and Romania normally needed a work permit to work in the UK. In addition to this, the types of work that Romanian and Bulgarian nationals were allowed to do were limited.
Romanians and Bulgarians who were low-skilled workers were restricted to schemes in the agricultural and food processing sectors. Skilled workers were able to come to the UK if they had work permit or qualify for the Highly Skilled Migrant Programme. With limited exceptions, Bulgarian and Romanian nationals have been required to obtain a 'worker authorisation document' before beginning employment in the UK.
Now that restrictions have been lifted, Romanian and Bulgarian nationals are subject to the same rules as most other EU nationals, ie they are able to come to work in the UK on the same basis as other EU countries.