Rivals say Google could reinforce its dominance unless EU regulators win more concessions from the Internet search giant over accusations it restricts consumer choice and competition.
Google last month offered to mark out its services from rival products in Internet search results and provide links to at least three competing search engines to end a 30-month investigation by the European Commission.
It said specialized websites will be able to opt out from the use of all their content in Google's own specialized search services and prevent the company from using specific information such as addresses and opening hours.
The EU antitrust watchdog said it would assess feedback from Google's rivals during a one-month test which started on April 26 before deciding whether to accept the company's proposal.
British price comparison site and Google complainant Foundem said its initial analysis showed that the offer failed to address the concerns raised by more than a dozen rivals.
"The proposed changes would make things considerably worse," the company said in a statement.
"Many of the proposals seek to legitimize its existing abusive practices, while others seek to secure permission for further abuses that Google might otherwise have been understandably nervous about introducing," it said.
German online mapping service Hotmaps was equally critical.
"The suggestion to let rivals bid and pay for special 'rival links' is unacceptable. If Google has anti-competitively harmed rivals by demotion and self-preferencing, it needs to remedy the damage done and not be paid to do so," said Hotmaps CEO Michael Weber.
Google spokesman Al Verney said the company continues to work co-operatively with the Commission.
The stakes are high for Google as a fine for breaching EU rules could cost it as much as $5 billion or 10 percent of its global turnover.
(Editing by David Cowell)