Sir Tim Harrow handed the six-page Article 50 letter, written by May, to Tusk at 12:20pm in Brussels.

The letter states: ‘I hereby notify the European Council in accordance with Article 50(2) of the Treaty on European Union of the United Kingdom's intention to withdraw from the European Union.’

The letter begins by saying: ‘On 23 June last year, the people of the United Kingdom voted to leave the European Union. As I have said before, that decision was no rejection of the values we share as fellow Europeans. Nor was it an attempt to do harm to the European Union or any of the remaining member states.

‘On the contrary, the United Kingdom wants the European Union to succeed and prosper. Instead, the referendum was a vote to restore, as we see it, our national self-determination. We are leaving the European Union, but we are not leaving Europe - and we want to remain committed partners and allies to our friends across the continent.’

In her letter, May sets out her desire to start negotiations between the UK and the EU as well as outlining proposed principles and the task ahead. 

To make the process smooth May sets out several principles for discussion with the European Council, these include:

• engaging constructively and respectfully, in a spirit of sincere cooperation;
• putting citizens first;
• working towards securing a comprehensive agreement;
• working to minimise disruption and provide certainty;
• acknowledging the importance of the UK's relationship with Ireland;
• prioritising the biggest challenges; and
• continuing to work together to advance and protect our shared European values.

During Prime Minister’s Questions May said that government is now ‘setting in process the future for this country and negotiations of how to work with the EU’.

May said: ‘This is an historic moment from which there can be no turning back - Britain is leaving the European Union – we are going to make our own decisions and our own laws, we are going to take control of the things that matter most to us and we are going to take this opportunity to build a stronger and fairer Britain that our children and grandchildren are proud to call home.’

Leaving the EU is expected to be a two-year process, with the UK will leaving the EU officially on 29 March 2019, with or without an agreement.

On 30 March the Great Repeal Bill will be published with Tusk publishing the European Council’s negotiation guidelines on 31 March.

The Bill is likely to reach royal assent in early 2018 with both houses of parliament, the European Council and parliament having a vote on any Brexit deal in late 2018 to early 2019.

May said: ‘Now that the decision to leave has been made and the process is underway it is time to come together…let us come together and work together, let us together choose to believe in Britain with optimism and hope.’

Theresa May's Article 50 letter to President Donald Tusk is available here.