BRITAIN looks set to leave the European Union by summer 2019 after triggering the formal process to pull out before the end of March next year, Theresa May has said.

The prime minister said Article 50 of the Lisbon Treaty will be triggered in the first three months of 2017, marking the start of the two-year process to enact Brexit.

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The process can be extended beyond two years if Britain and all other EU countries unanimously agree, but that prospect is seen as unlikely.

She made the announcement after revealing plans for a "Great Repeal Bill" to transpose all EU law applying to the UK into domestic law, ready for the day the country leaves the union.

Ahead of her speech on Brexit at the Conservative Party conference in Birmingham, Mrs May told BBC One's Andrew Marr Show: "As you know, I have been saying that we wouldn't trigger it before the end of this year so that we get some preparation in place.

"But yes, I will be saying in my speech today that we will trigger (Article 50) before the end of March next year."

Mrs May added: "The remaining members of the EU have to decide what the process of negotiation is.

"I hope, and I will be saying to them, now that they know what our timing is going to be - it's not an exact date but they know it will be in the first quarter of next year - that we'll be able to have some preparatory work so that once the trigger comes we have a smoother process of negotiation.

"It's not just important for the UK but important for Europe as a whole that we're able to do this in the best possible way so we have the least disruption for businesses, and when we leave the EU we have a smooth transition from the EU."

The prime minister was challenged on how she will seek to control immigration post-Brexit.

Asked if a work permit system would be adopted for skilled workers, Mrs May said: "We will look at the various ways in which we can bring in the controls that the British people want, and ensuring, as we have been in our immigration policy generally, that the brightest and best can come to the UK."

Tory former minister Anna Soubry, who is on the liberal pro-Europe wing of the party, said triggering Article 50 so soon "really concerns" her and warned that the EU "holds all the cards" in the negotiation.

She warned that companies like Nissan will "pack up and leave" Britain if it cannot negotiate access to the single market and dismissed the repeal Bill as "very technical and not a big deal".

Ms Soubry told Peston: "Triggering Brexit as early as March really concerns me, troubles me hugely, because we won't have had the French elections, we won't have had the German elections, and I'm sorry, it is going to take a lot of time and effort to disentangle ourselves and get the right deal."