More than three-quarters of European countries use American cloud computing services for vital national security functions, a report published on April 17 found.
The analysis arrives as European governments increasingly fear that vital digital services could be subject to an American "kill switch" if tensions with President Donald Trump's White House escalate.
"Russia and Putin are waging a war against a European country in Ukraine... but we also had a U.S. President threatening Denmark and Greenland," said Tobias Bacherle of the Brussels-based Future of Technology Institute (FOTI).
"This geopolitical shift needs to be taken in account," Bacherle said.
National security systems in 23 of the 28 countries studied "seem to rely on U.S. tech", researchers found after looking at EU member states and Britain.
The researchers analysed public information from defense ministry websites, national media and EU and U.K. public procurement records to identify major cloud computing contracts with American providers including Microsoft, Google, Amazon and Oracle.
Of the countries studied, 16 of 28 "are at high risk to a potential U.S. 'kill switch'", FOTI said, among them Germany, Poland and Britain.
Just one, neutral Austria, which is not a NATO member, was classified as lower risk.
Some capitals have already sought domestic or European options for tech procurement, prompting American firms to offer "sovereign" cloud computing services that they say are beyond Washington's reach.
But "This branding label fails to address the underlying dependencies", FOTI said, warning that such systems could still fall under the CLOUD act that allows American authorities to demand that companies hand over data stored abroad.
American companies could also be forced to cut off maintenance and security updates by U.S. sanctions, the researchers added.
