Prime Minister Theresa May is to officially notify the European Union next Wednesday that the UK is leaving.
Downing Street 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.
An EU spokesman said it was "ready and waiting" for the letter.
Mrs May's spokesman also rejected reports an early election may be held, saying: "It's not going to happen."
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.