Regarding your current status before the implementation of Brexit on Dec. 31, 2020, you are considered as EU citizen. Before the transmission period expires, you have to apply for a permanent resident certificate for an EU citizen, which will provide you specific benefits for the near “Brexit future." You have the ability to apply before the local Decentralized Administration for an initial 2-years resident permit after the Jan. 1, 2021. This permit is renewable for three years every time. In the meantime, under this status, you could be able to work in Greece legally.
