Here's the list of companies complaining to the EU about Google's dominance of the ad tech market

Several of Google's ad tech rivals - including Microsoft, WPP's AppNexus, and OpenX - have asked European regulators to investigate the company's dominance of the online display advertising space, The Financial Times reports.

The companies visited the European Commission in recent months and submitted documents in the hope of prompting an antitrust probe, although they have yet to file a formal complaint, according to the report.

Business Insider has contacted Microsoft, AppNexus, OpenX, and Google for comment, and we will update this article once we hear back.

At the heart of the issue, according to The Financial Times, is the way that Google apparently "ties and bundles" its ad tech products together, which rivals worry may discourage potential customers, or even prevent them from using their services.

Back in December, Digiday reported that media executives were concerned Google was attempting to "coerce" them into using two of its products - DoubleClick Bid Manager, its demand-side buying platform, and DoubleClick Ad Exchange -together. A Google spokeswoman told Business Insider shortly after this report was published: "In no way do we require clients to buy Ad Exchange inventory through DoubleClick Bid Manager (and in fact, many don't.)"

However, were this to be the case, it would difficult for executives at ad tech companies to go on the record about any form of apparent "tying" because many of the companies which are competitors with Google are also customers of Google. WPP CEO Sir Martin Sorrell has famously several times referred to Google as a "frenemy."

People close to the commission confirmed to the Financial Times that they have received commercial information, supposedly from the companies mentioned in the report, but said the ad tech industry is not the "specific focus" of any of their investigations.

Google does not give much detail on the size of its various ad tech businesses, but last quarter total advertising revenues grew 11% year on year to $16 billion, largely driven by search ads. Pivotal Research analyst Brian Wieser estimates that Google's DoubleClick business generated $2.1 billion in gross revenue last year, while its Google display network is estimated to have generated $6.9 billion.

In the US, Google will take a 13% share of the $27 billion digital display ad market in 2015, behind Facebook with a 25.2% share, according to eMarketer.

Were European regulators to investigate Google's dominance of the ad tech market, it would be just the latest in a series of antitrust probes into the company.

In April, the European Commission launched a formal investigation into Android, in addition to an update into its separate probe into Google's alleged abuse of its dominance in search. Google denies the charges.

Back in 2013 the Federal Trade Commission kicked off an antitrust investigation into Google's display advertising practices, but it was closed last year without any charges being leveled at the company, The Financial Times notes.