Theresa May has vowed to “negotiate hard” as Downing Street announced Article 50 would be triggered on 29 March.
The official notification will be sent to European Council President Donald Tusk next Wednesday, starting the formal process of leaving the EU.
The move will pave the way for two years of negotiations, with the country expected to be out of the bloc by the end of March 2019.
Speaking during a visit to Swansea, Mrs May said: “I am very clear that I want to ensure we get the best possible deal for the United Kingdom.”
The PM promised to work on a deal “that works for everyone across the United Kingdom and all parts of the UK”.
“I have set out my objectives,” she said. “These include getting a good free trade deal. They include putting issues like continuing working together on issues like security at the core of what we are doing.
“We are going to be out there, negotiating hard, delivering on what the British people voted for.”
The Article 50 announcement came after Britain’s envoy to Brussels, Sir Tim Barrow, informed Mr Tusk’s office on Monday morning.
Brexit Secretary David Davis said triggering Article 50 would initiate “the most important negotiation for this country for a generation”.
He said the Government wanted to secure “a new, positive partnership between the UK and our friends and allies in the European Union”.
The EU is expected to release an initial response to the Article 50 notification within 48 hours.
The remaining 27 leaders are also expected to call an extraordinary summit within four to six weeks to give a mandate for European Commission chief negotiator Michel Barnier, with talks probably beginning in earnest in May or June.
Mr Tusk tweeted that he plans to present draft Brexit guidelines to the remaining 27 member states within 48 hours of notification.
The negotiations will cover a wide range of subjects and seek to disentangle Britain after more than 40 years of integration.