The prime minister said it was a “unique opportunity” to “shape a brighter future” for the UK.
She was speaking after Britain’s EU ambassador formally triggered the two year count down to the UK’s exit by handing over a letter in Brussels.
It follows June’s referendum which resulted in a vote to leave the EU.
In a statement in the Commons, the prime minister said: “Today the government acts on the democratic will of the British people and it acts too on the clear and convincing position of this House.”
She added: “The Article 50 process is now under way and in accordance with the wishes of the British people the United Kingdom is leaving the European Union.
“This is an historic moment from which there can be no turning back.”
She said Britain would now make its own decisions and its own laws and “take control of the things that matter most to us – and we are going to take this opportunity to build a stronger, fairer Britain, a country that our children and grandchildren are proud to call home”.
She told MPs that this marks “the moment for the country to come together”.
Mrs May promised to “represent every person in the whole United Kingdom” during the negotiations – including EU nationals, whose status after Brexit has yet to be settled.
“It is my fierce determination to get the right deal for every single person in this country,” she said.
“For, as we face the opportunities ahead of us on this momentous journey, our shared values, interests and ambitions can – and must – bring us together.”
The six page letter from Mrs May triggering Article 50 of the Lisbon Treaty was handed over to EU Council President Donald Tusk by the UK’s ambassador to the EU Sir Tim Barrow.
It called for a “deep and special” partnership in the best interests of both the UK and the EU – but said the UK wanted to agree a trade deal alongside a withdrawal deal within the two year time table allowed by Article 50 of the Lisbon Treaty.
In a brief statement, Donald Tusk said the EU’s aim in negotiations was “damage limitation” and to “minimise costs” for EU citiziens, businesses and member states.
More details: http://www.bbc.com/news/uk