The ECA gives effect to the UK’s obligations as a member of the EU and makes EU law supreme in the UK. The ECA will be repealed on the day we leave the EU, returning power to the UK Parliament.
Source law
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/604516/Great_repeal_bill_white_paper_accessible.pdf
Annex A, A.2