The source, who is familiar with the Brexit negotiations, warned that Brussels would not be able to fully endorse an Article 50 extension without consultations in member states.
“The letter has come at a rather late moment in time, maybe too late, to take all kinds of decisions,” the diplomat said.
“EU cabinets and Parliaments have not been able to discuss the letter.”
Concerns have also been raised with the contents of Mrs May’s letter to Donald Tusk, the European Council president, setting out her plan to delay Brexit until June 30.
The Prime Minister claimed she will use the extra time to put her draft EU withdrawal agreement to MPs one more time after some “domestic” promises on the Irish backstop.
“We are happy there is a letter. We will go to great lengths to avoid chaos on Friday night next week, anything that keeps us avoiding chaos is welcomed” the diplomat said.
“We would have wished to already have a message from the House of Commons that they are going to pass the agreement they deal that is struck and that clearly is not there yet.”
EU27 leaders will seek extra “clarifications” from Mrs May on elements of her letter to Mr Tusk ahead of tomorrow’s European Council summit.
“We might have to get back to the negotiating table with the EU27 to go forward in a way that we don’t push Europe into chaos at the end of next week,” the diplomat added.
In her letter to Mr Tusk, the Prime Minister writes reveals she will table a motion on the Withdrawal Act 2018, which enshrines Britain’s EU departure date in law, in order to delay the divorce.
She adds: “If the motion is passed, I am confident that Parliament will proceed to ratify the deal constructively.
“But this will clearly not be completed before March 29 2019. In our legal system, the Government need to take a Bill through both Houses of Parliament to enact our commitments under the withdrawal agreement into domestic law.
“While we will consult with the opposition in the usual way to plan the passage of the bill as quickly and smoothly as possible, the timetable for this is inevitably uncertain at this stage.”
In her note to Brussels, Mrs May rules out a lengthy extension and insists it is not in Britain’s interest to hold European Parliament elections in the country.
“I do not believe that it would be in either of our interests for the UK to hold European Parliament elections,” she said.