Brexit: Theresa May announces date for triggering Article 50 to leave the European Union

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

The minimum period for leaving the European Union after article 50 has been triggered is two years.

Any country wishing to leave the EU is free to do so but must invoke Article 50 to negotiate their future relationship with the union.

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

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 also 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.