As the European Union’s antitrust probe into Google lurches ahead into its third year, a group of eleven companies opposed to the search engine giant have decided to step up the pressure, writing a letter to regulators urging them to file formal charges against the company instead of pursuing a settlement.

For the last two months, the European Commission has been examining a proposal volunteered by Google to alleviate worries over its alleged anticompetitive practices. Google’s rivals, which include Microsoft, TripAdvisor, Expedia and others, have accused the search engine of manipulating its rankings in order to display its own products and services first, as part of a larger effort to monopolize sectors of the internet economy that it retains an interest in. In addition, Google has been accused of ripping off travel and restaurant reviews from rival sites, and placing unfair restrictions on advertisers using its services.

If Google is unable to reach a settlement with the EU, it’s likely to find itself taken to court. Should it then be found guilty of engaging in anticompetitive behavior, Google could be liable for a fine of up to $5 billion.

Since Google handed over its settlement proposal in late January, we’ve heard next to nothing from the European Commission, leading to speculation that some kind of deal is being ironed out behind the scenes. One rumor that did emerge is that Google offered to differentiate its results from those of competing companies by labeling them as Google services, though whether or not this would actually have much impact click-throughs is open to question.

That may or may not be true, but it’s clearly not what Google’s rivals are hoping for. In a letter to the European Commission today, Google’s critics have argued that any settlement is unlikely to put an end to its anticompetitive practices, pointing to its delaying tactics as proof that it will try to find ways to circumvent whatever it agrees too.

“Google’s past behavior suggests that it is unlikely to volunteer effective, future-proof remedies without formally being charged with infringement,” said the group in its letter to Joaquin Almunia, Chairman of the European Commission.

“Given this, and the fact that Google has exploited every delay to further entrench, extend and escalate its anticompetitive practices, we urge the Commission to issue the statement of objections.”

Known as an “SO”, the statement of objections is an official document issued by EU regulators that lays out the issues that companies need to address in order to satisfy anticompetitive laws. Any company that fails to take action after receiving an SO is liable to a multi-million dollar fine, and could even be ordered to stop its anticompetitive actions.

So far, neither Google nor the EU has commented on the letter.