Thousands of EU laws on everything from workers' rights to the environment are to be scrapped or replaced with UK equivalents in a new plan.
Details of the planned Great Repeal Bill are to be published shortly - the day after the UK officially began the two year process of leaving the EU.
Ministers need to "copy and paste" EU laws into UK law to avoid a legal "black hole" when Brexit happens.
It comes as ministers reject claims of a row with the EU over future security.
In her letter on Wednesday triggering Article 50 of the Lisbon Treaty, Mrs May suggested cooperation with the EU on the fight against crime and terror was at risk if Britain did not agree an overall Brexit deal within the two year time limit.
The prime minister's words sparked an angry reaction from some EU officials who said security was too important to be used as a bargaining chip in trade negotiations.