Stubb considers the claims to be a serious matter.
"You shouldn't spy on your buddies," he said in an interview with Yle.
European Parliament Deputy Speaker Isabelle Durant on Sunday raised the prospect of putting off scheduled free trade talks between the US and the EU because of the spying allegations.
"In the free trade negotiations we know precisely what we think of each other and what our positions are. This is a matter of trust. If trust is broken, the consequences are serious," stated Stubb.
Harsh words to be coming from Finland