Theresa May will announce she will repeal the 1972 European Communities Act in a move that will formally begin the process of making Britain's Parliament sovereign once again.
Addressing the Conservative Party Conference for the first time as leader, Mrs May will declare that her Government will begin work to end the legislation that gives European Union law supremacy in Britain.
In its place, a new "Great Repeal Bill" will be introduced in Parliament as early as next year to put power for the nation's laws back into the hands of MPs and peers.
The announcement is Mrs May's first firm commitment on Brexit since becoming Prime Minister in July and marks a major step on the road to ending the country's EU membership.
Leading Eurosceptics are likely to cheer the news after they put repealing the law at the heart of a "Brexit manifesto" published just days before the referendum.
It is intended to show critics that No 10 does have a plan for Brexit, after weeks of sniping that the Government does not have a clear strategy for the forthcoming negotiations.Ministers will also announce protections for workers' rights secured via Brussels, such as parental leave and automatic holiday, to pre-empt Labour attacks.
Mrs May will take to the stage with her three Brexit Cabinet ministers - Boris Johnson, Liam Fox and David Davis - to show a unified front on the first day of conference.
She said on Saturday night: "We will introduce, in the next Queen's Speech, a Great Repeal Bill that will remove the European Communities Act from the statute book. That was the act that took us into the European Union.
"This marks the first stage in the UK becoming a sovereign and independent country once again. It will return power and authority to the elected institutions of our county. It means that the authority of EU law in Britain will end."
Mr Davis, who is charged with leading the negotiations as Secretary of State for Exiting the EU, will explain the decision during his speech.
"EU law will be transposed into domestic law, wherever practical, on exit day," he will say. "It will be for elected politicians here to make the changes to reflect the outcome of our negotiation and our exit. That is what people voted for: power and authority residing once again with the sovereign institutions of our own country."
The European Communities Act 1972 allowed Britain to join what would become the EU the following year. It also enshrined the supremacy of EU law in the UK, making the European Court of Justice [ECJ] the ultimate arbiter in legal disputes.
Throughout the years, as controversial judgments from the ECJ often triggered anger among Tory MPs, the legislation became symbolic of Brussels's influence over Britain.
Vote Leave, the formal campaign to leave the EU, named repealing the European Communities Act as one of their six Brexit "road map" promises a week before the vote.
The group was headed up by Boris Johnson and Michael Gove. The new "Great Repeal Bill", introducing the change, will be included in next year's Queen's Speech in spring 2017.
The "historic" piece of legislation will allow Parliament to write parts of EU law it wants to keep into the British system while discarding unwanted elements. Government sources hope the move will show that ministers want to give Parliament a say on the Brexit process and will open negotiations up to parliamentary scrutiny. However, the process is not without risk.
A majority of both MPs and peers will need to vote for the Bill for it to pass, raising the prospect pro-EU Lords could hold up its progress.
Speaking on Sunday, Mr Davis will move to assure workers concerned that key rights which were introduced on an EU-wide level will remain in place.
"To those who are trying to frighten British workers, saying 'When we leave, employment rights will be eroded', I say firmly and unequivocally 'no they won't'," he will state.
The legislation will only come into effect on the day that Britain leaves the EU - expected to be as early as 2019, two years after Mrs May formally begins the negotiations.
Mrs May's speech at 2pm will open the Conservative Party's four-day annual conference in Birmingham. Despite growing pressure from Eurosceptic Tory backbenchers and EU leaders, she is not expected to name a date for triggering Article 50, the mechanism to start talks.
She is also expected to steer clear of the "hard Brexit" and "soft Brexit" labels which have emerged as a way of defining where people stand on the negotiations in recent days.
She will use the speech to emphasise that her Government has a plan to fulfil the vote for Brexit in June that triggered the upheaval that let her enter No 10.
The conference will also give Mrs May a platform to better explain her vision for her premiership after a summer largely bereft of major policy announcements.
Earlier this week Ken Clarke, the long-serving Tory MP who held posts in three prime minister's cabinets, criticised Mrs May for running a "government with no policies".
Anonymous briefings about the tight control she holds on policy and willingness to clash with other Cabinet ministers have also surfaced. Aides rebut the criticism but say Mrs May will spell out her vision for social change and an economy that works for "everyone" during the conference.