Google's rivals have indicated they will reject its offer to label its services in search listings, as it tries to settle a long-running antitrust investigation by the European Commission over its market dominance.

The US search company's offer was publicised on Thursday by EC competition commissioner Joaquín Almunia, who has given rivals and outside organisations a month to comment on the proposals, which aim to settle an ongoing antitrust tussle with the EC.

If organisations broadly accept the ideas, then Google will be bound over to obey them and display the results in a set format in countries within the EC for five years and three months.

But if Google's suggestions to settle are turned down – as looks likely – Almunia could be obliged to issue a formal "Statement of Objections" to Google's conduct in search and force it to follow a legally binding code of conduct, or face swingeing fines of up to 10% of its global revenues.

Google has been under investigation since November 2010, when Almunia's office opened an investigation into Google's dominance of search following complaints by the UK "vertical search" company Foundem, Microsoft-owned Ciao, and the French legal search engine ejustice.fr.

A similar investigation into Google's search dominance in the US fizzled out earlier this year, when the Federal Trade Commission made no move to enforce any changes on Google's search results layout.

The independent European consumer organisation BEUC, whose members include Which? in the UK, said in a statement it was disappointed with the proposals, which it said would not achieve the aim of "eradicating the current anti-competitive behaviour in what is essentially a monopoly market". Google has over 90% of search share in Europe, compared to about 65% in the US.

Google's suggestions were set out after more than nine months' negotiation with the EC. In 2011 Almunia's team had raised a series of concerns over how Google labels its search results, "scrapes" pages on other sites to display in search listings, requires exclusivity deals on its adverts on web pages, and prevents easy portability of advertising campaigns to other search engines.

The biggest concern is over what is perceived as Google's promotion of its own services, such as Maps, YouTube, Shopping and Local. The company's proposals, unveiled by Almunia, suggest that it would label listings where it is promoting products on its own behalf. In some cases, it would also offers links to up to three other "vertical" rival sites, decided by a test of how many page views those sites have received in the European economic area.

But BEUC said the "labelling" proposal "may even shepherd consumes towards clicking on Google services [that would be] highlighted in a frame … Labelling an infringement of competition law doesn't prevent it being an infringement."

Instead, opponents say, Google should be forced to compete in the same way that they are – on the basis of their content and Google's indexing system.

Shivaun Raff, co-founder of the UK vertical search company Foundem – one of the original complainants to the EC in spring 2010, after alleging that the US company was artificially demoting it in search results – said: "The only foolproof way to tackle abusive practices is to end them. Ultimately, the only way to end Google's search manipulation practices and restore a level-playing field is to ensure that Google holds all services, including its own, to exactly the same standards, using exactly the same crawling, indexing, ranking, display, and penalty algorithms."

A Google spokesman said: "We continue to work co-operatively with the European Commission."