Bill Clinton has suggested some of those who voted for Brexit may only now be realising the impact of their decision.
The former US president made the comments as he was awarded an honorary doctorate at Dublin City University for his work on peace and reconciliation in Northern Ireland.
"I'm sorry we can't stay together, we had a disagreement," he said as he imagined the thoughts of a Leave voter.
"Oh my God, I didn't know I was going to lose that customs [union] thing and all these economic benefits. Why didn't anyone tell me that?"
In his acceptance speech, Mr Clinton warned that the Brexit vote was driven by people thinking differences are more important that what they have in common.
"Given the economic inequalities and the rapid pace of social change and all the upheaval that's going on .... people are reassessing whether what we have in common is more important than our differences.
"A lot of people begged to differ.
"That's really what the Brexit vote is all about."
Mr Clinton raised concerns over the of nationalist parties in a number of European country as he warned that many were retreating to a "tribal" identity where they no longer wished to be part of a wider community.
He added: "All partnerships that are community-based are held together not because everybody agrees with everybody else, not because we don't still have our particular identities, but because co-operation is better than conflict or isolation in any environment in which you must be in touch with others.
"It's a simple proposition. But we are re-litigating it now."