German Foreign Minister Frank-Walter Steinmeier said negotiations should begin as "soon as possible".
He made the comments after an urgent meeting of the six EU founder members to discuss the decision.
British PM David Cameron has said he will step down by October to allow his successor to conduct talks.
The six countries attending the summit in Berlin - Germany, France, Italy, Belgium, Luxembourg and the Netherlands - first joined forces in the 1950s and still form the core of the EU.
