Berlin and Paris on Monday called for an explanation from Copenhagen after a report that Danish authorities helped the US National Security Agency (NSA) wiretap some of their senior politicians.
"I want to say that this is unacceptable between allies. That's clear," said French President Emmanuel Macron after a meeting of the Franco-German council of ministers.
German Chancellor Angela Merkel, who had been among those wiretapped, said she could "only agree."
Macron demanded "complete transparency and resolution of the matter by our Danish and American partners."
Earlier on Monday, the Danish government distanced itself from certain spying practices, after investigative journalists reported that Denmark allegedly supported the United States' wiretapping of high-ranking European politicians about a decade ago.
"Systematic wiretapping of close allies is unacceptable," said Danish Defence Minister Trine Bramsen.
Merkel said she was "relieved" that the Danish government clearly expressed its attitude towards the allegations. "In this regard I see a good basis not only for the resolution of the matter, but also to really come to trusted relations."
Investigative journalists from Danish broadcaster DR, together with German media outlets like NDR, WDR and the Sueddeutsche Zeitung, on Sunday revealed that the NSA had allegedly wiretapped leading politicians' phones with the help of software created by the Danish military intelligence service FE.
Swedish, Norwegian leaders
The research was based on anonymous sources and an internal analysis of the FE, from 2012 and 2014.
Aside from Merkel, other German politicians - such as President Frank-Walter Steinmeier as well as leaders in France, Sweden and Norway - were targeted.
The investigation revealed for the first time the extent of alleged military intelligence collaboration between Denmark and the US, and expanded the list of names of the politicians targeted.
Bramsen said she could not comment on speculation about possible intelligence activity. FE also did not comment on the allegations.