Former French government minister and ex-European Commission vice-president Michel Barnier has been appointed as the commission's chief negotiator for Brexit.
The appointment was announced by commission president Jean-Claude Juncker, who said he wanted "an experienced politician for this difficult job".
Mr Barnier officially takes his position on October 1 and will spend the coming months preparing the ground in Brussels for the negotiations, but will not make contact with UK authorities until the two-year process of withdrawing from the European Union is formally triggered by London under Article 50 of the Lisbon Treaty.
The appointment comes days after Mr Juncker acknowledged that the UK Government may need several months to prepare its position before negotiations start.
In the immediate aftermath of the EU referendum on June 23, the Commission president indicated that he wanted Article 50 to be activated immediately to allow a swift UK withdrawal to limit uncertainty.
But in a French TV interview on Monday, he said he had no "deadline" for the talks to begin, adding: "The British Government needs several months to fine tune its position. Our British friends know that there will be no negotiation before notification of their farewell letter."
However, he stuck to his hard line on the outcome of the negotiations, insisting that Britain will have to accept EU rules - including on freedom of movement - "without exception or nuance" if it wants to keep full access to the single market after Brexit.
Mr Barnier will report directly to Mr Juncker as head of the Commission Taskforce for the Preparation and Conduct of the Negotiations with the United Kingdom.
He will have the rank of Commission director-general and will be able to draw on the advice of Brussels' most senior civil servants and experts.
Describing Mr Barnier as "a skilled negotiator with rich experience in major policy areas relevant to the negotiations", Mr Juncker said: "I am very glad that my friend Michel Barnier accepted this important and challenging task. I wanted an experienced politician for this difficult job.
"He has an extensive network of contacts in the capitals of all EU member states and in the European Parliament, which I consider a valuable asset for this function.
"Michel will have access to all Commission resources necessary to perform his tasks. He will report directly to me, and I will invite him to brief regularly the College (of commissioners) to keep my team abreast of the negotiations. I am sure that he will live up to this new challenge and help us to develop a new partnership with the United Kingdom after it will have left the European Union."
In a message on Twitter, Mr Barnier said: "Honoured to be entrusted with UK negotiation by @JunckerEU and @EU_Commission. Rendez-vous for beginning of demanding task on 1 October."
A former MP and MEP in the centre-right UMP party, Mr Barnier served as minister for the environment, European affairs, foreign affairs and agriculture under presidents Jacques Chirac and Nicolas Sarkozy and was a European Commissioner from 1999-2004 and 2010–14.