In a wide-ranging interview, de Villepin also appeared to suggest that ramping up military activity in Iraq and Syria, as French President Francois Hollande promised to do after the attack in Nice on Friday, may not be the right response to Europe's terror threat.
Theresa May, who replaced David Cameron on Thursday as Britain's prime minister, lacked a strong sense of urgency and had "no idea of what she should do" in response to the U.K's vote to leave the European Union (EU), de Villepin told CNBC's Eunice Yoon on the sidelines of a conference in Beijing on Monday.
"This is always the problem. People get into a position of responsibility without having the knowledge, without being prepared," he said.
"The truth is we have been through this referendum in the U.K. and nobody has been exploring [the issue] of what [would happen] if [people voted for a] Brexit. So today it's an unprepared situation on both sides."
Both the U.K. and Europe needed to find a solution to the consequences of Brexit to avoid adverse consequences to economy, he said.