The spokesperson added that Downing Street expects a response from European Union up to 48 hours afterwards.
May has previously said that she would start the Brexit process "by the end of March", therefore she has met her commitment.
Speaking after London announced that Prime Minister Theresa May would start the two-year withdrawal process by writing to European Union summit chair Donald Tusk next Wednesday, the source said this did not leave enough time to convene the other 27 leaders on April 6-7, dates that had been pencilled in for a meeting.
Last week, Juncker expressed in a statement to the European Parliament that he is hoping for a constructive process "conducted in an orderly manner".
EU leaders will meet in late April or early May to approve Brexit negotiating guidelines after Britain triggers its divorce, a European source said today. By waiting until Wednesday it allows May to brief the cabinet on Tuesday and be grilled by MPs at prime minister's questions on Wednesday afternoon. After triggering Article 50, the United Kingdom can begin negotiating terms of the departure from the organization and iron out any future relations with other states in the EU.
Man United to face Anderlecht in Europa League quarterfinals
Juan Mata has said that "it is not by chance" that Manchester United are now unbeaten in their last 18 Premier League matches . Chelsea is ten points ahead second place Tottenham, and they keep demonstrating every week there is no stopping them.
News of the Article 50 date - known as "B Day" among expectant Euroskeptics - was a closely guarded government secret.
According to BBC News, 10 Downing Street officially announced May's intention to write and submit a formal letter to the European Union announcing her invoking of Article 50.
Secretary of State for Exiting the EU, David Davis, also said May would trigger the Brexit legislation next week.
"We are on the threshold of the most important negotiation for this country for a generation", he added. "That's why we believe the people should have the final say over the Conservative Brexit deal".