British Prime Minister Theresa May has signed the letter that when delivered tomorrow will formally begin the Brexit process.

It will be received by the President of the European Council Donald Tusk in Brussels.

The notification triggers Article 50, and marks the start of two years of negotiations.

At around the same time the prime minister will address the British parliament calling for the country to "come together" during negotiations.

Sky News political editor says Mrs May has already been talking to European leaders ahead of the formal farewell: