The treaties are the primary source of EU law. A substantial proportion of the treaties sets out rules for the functioning of the EU, its institutions and its areas of competence. While much of the content of the treaties will become irrelevant once the UK leaves the EU, the treaties (as they exist at the moment we leave the EU) may assist in the interpretation of the EU laws we preserve in UK law.

High

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/604516/Great_repeal_bill_white_paper_accessible.pdf

Chapter 2, Paragraph 2.9