The European Commission - which will represent the 27 countries in the negotiations with the U.K., has tried to dismiss comments of division within the bloc and pledged to speak with one voice.
However, its main negotiator admitted that Wednesday was just the first day of a "very difficult and long road".
Michel Barnier, the EU's negotiator, said on Twitter the executive is going to be "firm" on ensuring the rights of EU citizens during this process.
In a timeline explaining Brexit, the commission admitted the actual negotiating time is roughly 18 months - not the two years that the so-often-mentioned Article 50 states. This is due to EU proceedings, which include the need to take the final deal to parliamentary ratification.
There are increasingly more doubts that the whole process is going to be concluded before midnight on March 29, 2019. European lawmakers have opened the door to a transitional deal after that date "to ensure that custom controls and barriers on trade are not enforced on day one of Brexit," but such arrangements may not last beyond three years, the Guardian reported.