Oettinger told Germany's daily Bild that he assumed June's referendum outcome in favour of Britain exiting the EU was “binding”.
“But it is possible that public opinion will tip if the economic situation in the wake of the Brexit vote worsens,” said Oettinger, who is Digital Economy Commissioner.
“In any case, I wouldn't place any major bets on Brexit.”
Oettinger predicted that the longer the British government waited to trigger Article 50 of the EU's Lisbon Treaty – the legal process for leaving the bloc – “the more insecure the situation will become – economically and politically”.
Once Article 50 is triggered, it would start a two-year countdown to Britain's exit from the European Union.
Prime Minister Theresa May has said it will not be enacted this year, the government needing time to shape Britain's exit objectives first.
German Chancellor Angela Merkel has called for giving London the time it needs to plot the way forward and has refused to enter into preliminary negotiations before Article 50 is activated.