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.