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.

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.”

Mrs May said Parliament will be kept informed, adding: “This is not about keeping silent for two years, but it’s about making sure that we are able to negotiate, that we don’t set out all the cards in our negotiation because, as anybody will know who’s been involved in these things, if you do that up front, or if you give a running commentary, you don’t get the right deal.”

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.”

Mrs May will later tell the Tory party conference that her “Great Repeal Bill” will scrap the 1972 European Communities Act, which gives direct effect to all EU law, and at the same time convert Brussels regulations into domestic law.

This will give Parliament the power to unpick the laws it wants to keep, remove or amend at a later date, in a move that could be welcomed by MPs keen to have a say over the terms of Brexit.

The move is also designed to give certainty to businesses and protection for workers’ rights that are part of EU law.

Brexit Secretary David Davis will also tell the conference: “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’.”

The Bill is expected to be brought forward in the next parliamentary session (2017-18) and will not pre-empt the two-year process of leaving the EU, which begins when the Government triggers Article 50 of the Lisbon Treaty.

Mr Davis will say: “It’s very simple. At the moment we leave, Britain must be back in control. And that means EU law must cease to apply.

“To ensure continuity, we will take a simple approach. EU law will be transposed into domestic law, wherever practical, on exit day.

“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 repeal Bill will end the primacy of EU law, meaning rulings by the European Court of Justice will stop applying to the UK once the legislation takes effect.

It will include powers to make changes to the laws using secondary legislation as negotiations over the UK’s future relationship proceed, although more wide-ranging amendments or new laws may come forward in separate Bills.

Sir Craig Oliver, David Cameron’s former spin doctor, expressed his frustration over Mrs May’s stance during the EU referendum.

Sir Craig, who has released a book entitled Unleashing Demons: The Inside Story Of Brexit, told the BBC: “It was very difficult in the lead-up to that campaign having a Home Secretary not reveal which side she was on.

“When she did reveal which side she was on, it was 51-49 and was very equivocal.”

He added: “It’s perfectly legitimate for Theresa May to do that. What the book is doing is recounting what was it like being in Downing Street to be part of this tumultuous situation.”

Transport Secretary Chris Grayling said Mrs May was likely to give more information about the deal Britain was seeking before triggering Article 50 but that she would not be setting out her strategy in detail.

He would not be drawn on suggestions that the Government was leaning towards a “hard Brexit” outside the European single market.

On triggering Article 50, Mr Grayling told ITV’s Peston on Sunday: “Of course she will say more to Parliament before we reach that point but we’re not going to set out in detail our negotiating position before the negotiations start.”

He added: “What nobody would expect is for us to set out in detail before any negotiation exactly what the end point should be, because by definition that would make the negotiation something that was irrelevant.”

Leading Brexiteer Iain Duncan Smith suggested the process could be triggered before March.

The former Work and Pensions Secretary told Murnaghan on Sky News: “This is a ‘by the end of March’, so there is every chance she will be triggering it earlier than that.

“It depends hugely on what they are doing behind the scenes, trying to make sure they have exactly the areas that they want lined up.”

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.

“The other thing that’s got to be said is this – this idea that we hold the cards and that the EU is going to come to us and say ‘do you know what, we’ll give you pretty much what you want’, the idea we’re going to get anything like we’ve got now is rubbish.

“We’re going to get something worse, obviously we are, and we don’t hold the cards, the EU does.”

Labour shadow minister Jon Ashworth joined Ms Soubry in calling for more clarity over the Government’s Brexit strategy.

“Theresa May said she was providing ‘clarity’ but that’s exactly what we aren’t getting from the Tories,” he said.

“There were very few answers from her this morning either on the big questions facing us.

“She gave very little detail on her supposed big idea of a ‘Great Repeal Act’ other than it’s an ambition; there was no answer on what would be in it, how it would work or, vitally, how she intends to deliver Brexit while protecting our workers and businesses.”