"There needs to be a notification by the country concerned of its intention to leave (the EU), hence the request to (British Prime Minister David) Cameron to act quickly," Moscovici, the EU's economic affairs commissioner, told French media.
"There is no reason to prolong the uncertainty," he added.
Under EU law, only the member state concerned can notify Brussels of its intent to quit.
The very day the referendum was held, Cameron announced he would resign by October, raising speculation over whether he would himself notify Brussels -- or whether he would leave that task to his successor.
"Everyone agrees on the fact that a three-month waiting period is too long. But after (the notification is submitted), there will be months, years of negotiations" to re-define Britain's ties with the EU, Moscovici said.
Despite voting to leave the bloc, Britain "remains a strategic country", he added.
Asked whether there could be a way back from Brexit, Moscovici said no.