Google’s rivals could get the chance to ramp up pressure for tougher concessions from the search giant when they are asked, possibly as early as this week, to study its proposals for ending a European Union antitrust investigation.

The world’s most popular search engine formally submitted a package of concessions to European regulators last week and three people familiar with the matter said on Monday these could be put to rivals this week in a market test, according to our source..

The proposals are intended to settle an investigation dating from November 2010 and stave off a fine that could be as high as $5bn if the company were found to have breached EU rules.

“It is possible the market test could start this week,” said one of the sources, who declined to be identified because of the sensitivity of the matter.

The spokesman for competition policy at the European Commission, Antoine Colombani, declined to comment. Google spokesman Al Verney said the company continued to work cooperatively with the EU antitrust watchdog.

Complainants against Google include Microsoft, online mapping services, smaller rival search engines across Europe, publishers, online travel sites such as TripAdvisor and Expedia, and price comparison sites.

Neither the commission nor Google has given details of the proposals. But people familiar with the matter have told Reuters these could include Google labelling its own services to differentiate them from rivals’, and also imposing fewer restrictions on advertisers.

Google’s opponents have already indicated labeling will not address their concerns.

“Labelling is a non-starter and may be worse than the existing situation,” said David Wood, a lawyer for online lobbying group ICOMP, whose members include Microsoft and four other complainants.

Any concessions the regulators accept from Google would be valid for five years in Europe and be monitored by a trustee to ensure compliance, one of the sources said.

“Rival services will be shown more prominently in search results, with three links to three different search engines,” the person said.

“Advertising space in Google Shopping will be reserved exclusively for vertical search sites,” the person said, referring to specialty search functions focused on specific topics.