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.
The European Council's president said on Twitter he will present draft Brexit guidelines to the other 27 EU member states within 48 hours.
The process will give a negotiating mandate to the EU's executive arm, the European Commission, with chief EU negotiator Michel Barnier going ahead with the process of scheduling talks with his British counterpart, Brexit minister David Davis.
But EU negotiators warn it could take two years just to settle the divorce terms; agreeing on a new relationship for the United Kingdom and the EU could take years longer.
Up to early 2019, Britain will remain a full European Union member and nothing will change beyond the exclusion of its representatives from some European Union meetings where the approach of the remaining 27 to Brexit is discussed. However, the prime minister's spokesman was adamant that Downing Street remained opposed to an election before 2020.
The PM's official spokesman said: "Earlier this morning, the UK Permanent Representative to the European Union informed the office of Donald Tusk that it is the UK's intention to trigger Article 50 on March 29". However, in this version of Brexit there is no deal on customs, meaning customs checks for British imports and exports, there is only a limited free trade deal between Britain and the European Union, meaning tariffs on many goods and services, and there are border checks with the Republic of Ireland.
May has previously said that she would start the Brexit process "by the end of March", therefore she has met her commitment. "We want negotiations to start promptly".
Britain and Germany are planning to sign a new defence pact after Theresa May triggers Article 50.
Asked if the devolved governments had been informed this notification was being given on Monday, the Prime Minister's official spokesman said: "We are informing you all essentially now".
May's tour will also take her to Scotland where the Scottish Parliament will be voting Wednesday after a two-day debate whether to back a new independence referendum to form a breakaway country outside of the United Kingdom.
The developments come as an independent think tank warned today that up to 15 new bills could be needed to deliver Brexit.
Priti Patel, the UK's worldwide development secretary, said: "What this shows is that we are a government that is very focused".