Green Party veteran Alexander Van der Bellen railed against the populism behind Britain’s decision to leave the EU as he went on the attack.
He described his recent election victory against far-right candidate Norbert Hofer as evidence that you “can win against populism” and urged other EU chiefs to follow his lead.
But when asked about countries wanting to quit the bloc he made a succession of extraordinarily undiplomatic remarks which will be interpreted as a direct attack on Brexit.
The Austrian chancellor Christian Kern, who has struck a more eurosceptic tone during campaigning, looked on nervously alongside EU chief Jean-Claude Juncker as the rant unfolded.
Mr Van der Bellen blasted: “You must be crazy to believe that the old fashioned national sovereignty of the 30s gains you more power for your own country than being a member of the union.
“You don’t have to be particularly intelligent I don’t think to understand that.”
The Austrian president also railed against the “tragic and inappropriate decision of the majority of UK voters to vote for Brexit” and said the result had “woken a lot of people up in Austria”.
At one point Mr Van der Bellen also alluded to his personal opinion on the issue of whether Europe should become a superstate, saying he did not want to describe meeting EU officials as a “foreign trip” because “we’re not really abroad when we’re in Brussels”.
Although his role is largely ceremonial, the remarks by the Austrian president will hit home because they are indicative of the tone EU states are set to take throughout the Brexit process.