That means that the same laws should apply the day after Britain leaves the European Union, scheduled for 2019, as the day before — to provide legal continuity. But there are more than 12,000 regulations in force, and thousands more pieces of British legislation either incorporating, or influenced by, European law.

Some will need only minor alteration, for example, removing references to European regulators. In doing so, the government plans to deploy so-called Henry VIII clauses that gained their name from the Statute of Proclamations in 1539, which gave the king power to legislate by proclamation. Proponents say it will enable the government to change the details of some European laws when they are imported into the British statute book.

The use of the mechanism is intended to speed the process, by avoiding full scrutiny of each law and directive by Parliament. But the plan has provoked protests and complaints that Parliament, which an exit from the bloc was supposed to strengthen, is being circumvented.

The dispute also highlights the vast scale of the task faced by the British government as it unscrambles a morass of European Union lawmaking.