In most cases, the corrections made by the statutory instruments will need to be made before the UK leaves the EU, so that we have a functioning statute book on the day of the UK’s withdrawal. The Government intends therefore that the power in the Great Repeal Bill will come into force as soon as the Bill gains Royal Assent, so that the process of correcting the statute book can begin.

High

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

Chapter 3, Paragraph 3.24