The report comes amid concerns that some EU lawmakers who are reviewing the Digital Markets Act (DMA) proposed by EU antitrust chief Margrethe Vestager may be lenient towards cloud computing companies.

Amazon's Amazon Web Services was the leading provider in the second quarter, followed by Microsoft Azure and Google Cloud, market research company Statista found. Others include IBM Cloud, Alibaba Cloud, Salesforce and Oracle.

Frederic Jenny, chairman of the Organisation for Economic Cooperation and Development's Competition Committee, put together the study, in his personal capacity, for trade body Cloud Infrastructure Services Providers in Europe (CISPE).

The new EU rules should also cover software licensing, the report said, with respondents citing unfair terms imposed by some of the big software companies to access their cloud infrastructure.

"The DMA does say that cloud infrastructure can come within the ambit of the DMA but it is not obvious that all the suppliers are covered", Jenny said.

"For example, it doesn't seem that Google Cloud qualifies under the DMA as a gatekeeper or IBM Cloud or Salesforce."

Potential anti-competitive practices by some companies could include unfair pricing techniques or efforts to make users' move to a rival technically difficult, he said.

Jenny said the study interviewed some 25 companies that use cloud computing services, some of which cited issues such as unfair licence terms that force customers to pay again to use software they already own when they move to a competitor.

Respondents were also concerned about providers bundling software products with their cloud infrastructure to make rival products either less attractive or more expensive.

Google, Oracle and SAP declined to comment. Microsoft and Amazon did not respond.

EU lawmakers have to thrash out the draft DMA with EU countries before it becomes law, possibly in 2023.