The European Council's first President Herman Van Rompuy has predicted there will be no "substantive" Brexit talks for 12 months.
He reasoned such discussions will wait until Germany's new government is formed after elections in September 2017.
The former Belgian Prime Minister denied European leaders are looking to "punish" the UK for its decision to leave, but stressed negotiations will be difficult "independent of personalities".
He explained there are "huge economic interests" at stake but told the BBC: "There are also red lines. It is very well known that freedom of movement [of EU nationals] is a red line."
Mr Van Rompuy bluntly described the UK's referendum as "a historical mistake".
He said: "One of the rare examples where a country decides to vote against their own interests.
"For me, it was proof that referendums always gives the wrong answers to the question never posed."
Adding: "Of course it was not only about Europe, it was also about people feeling uncertain, people feeling anxious looking at the impact of globalisation."
The senior EU official said Mr Cameron was warned against the referendum in 2013 when he announced his intention to go through with it.
"We all told him [David Cameron] that this kind of referendum was a mistake", Mr Van Rompuy said.
Once the UK decides to trigger Article 50 of the Lisbon Treaty, there is a set two year period for Britian to conduct negotiations before leaving the EU entirely.
Prime Minister Theresa May has said she will not trigger Article 50 before the end of 2016, saying: "What is important is that we do this in the right timescale and we do it to get the right deal for the UK."
However, she is coming under pressure from senior Conservatives and Leave campaigners to begin Brexit negotiations as soon as possible
Former cabinet member and prominent Leave campaigner, Iain Duncan Smith MP said: "We shouldn’t wait to see the outcome of the two elections in Germany and France, that suggestion is yet another attempt to turn this referendum result into a neverendum."
