BBC UK – May said she would write a letter to the European Council, adding that it hoped negotiations on the terms of exit and future relations could then begin as quickly as possible.
The move comes nine months after a referendum in which the UK voted to leave by a margin of 51.9% to 48.1%.
An EU spokesman said it was “ready and waiting” for the letter.
Under the Article 50 process, talks on the terms of exit and future relations are not allowed until the UK formally tells the EU it is leaving.
If all goes according to the two year negotiations allowed for in the official timetable, Brexit should happen in March 2019.
A No 10 spokesman said the UK’s Ambassador to the EU, Sir Tim Barrow, informed the European Council, headed by President Donald Tusk, earlier on Monday of the date that Article 50 would be triggered.
Mrs May is expected to make a statement to the House of Commons on Wednesday shortly after invoking Article 50, setting out her aims.
A spokesman said the government wants negotiations to start as soon as possible but added that they “fully appreciate it is right that the other 27 EU states have time to agree their position”.
The BBC’s Ben Wright said he expected the Article 50 letter to be short, possibly extending to two pages at most, and for Mrs May to use it to publicly reiterate her general objectives – such as leaving the single market but reaching a mutually beneficial agreement on trade and other issues.
At the same Downing Street briefing for reporters on Monday a spokesman also dismissed suggestions of an early general election, saying: “It’s not going to happen.”
Speaking in Swansea on Monday, during the first of a series of visits around the UK before she triggers Article 50, Mrs May said she was intent on “delivering on Brexit and getting the right deal”.
Last year’s referendum result, she added, “was not just about leaving the EU” but was a vote for a “change in the way the country works”.
“Part of that is building a strong economy and ensuring that the benefits of economic growth and prosperity are felt across every part of the UK.”