Google’s job listing search tool has grown rapidly in popularity in recent months, but 23 rival job-finding sites are questioning the antitrust implications of the service.

Reuters reports that in a recent letter to European Union competition commissioner Margrethe Vestager, 23 job search websites based out of the E.U. raised concerns about the dominance of Google’s job search engine and requested that Vestager temporarily order Google to stop “playing unfairly” while she investigates the firm.

Google’s job search engine works similarly to Indeed.com, aggregating job posting and allowing users to apply for positions, save job postings, and get alerts for openings at specific companies or for particular roles. Google places ads for its job search tool at the top of search results for terms such as “call center jobs,” now rivals are worrying about the antitrust implications of these actions.

Rivals are alleging that Google is using its dominance in the online search world to attract users to its specialized job search tool without having to make traditional marketing investments. Vestager is reportedly examing Google’s job search engine. She will be leaving office on October 31 but has prepared an “intensive” handover to her successor who will likely investigate Google.

Nick Zakrasek, senior product manager for Google search, stated that the firm welcomes industry feedback on its jobs search engine. “Any provider — from individual employers to job listing platforms — can utilize this feature in search, and many of them have seen a significant increase in the number of job applications they receive,” Zakrasek said in a statement. “By improving the search experience for jobs, we’re able to deliver more traffic to sites across the web and support a healthy job search ecosystem.”

Google has previously faced antitrust issues in the European Union with the firm receiving a total of $9.3 billion in antitrust fines from the EU in recent years.

Lucas Nolan is a reporter for Breitbart News covering issues of free speech and online censorship. Follow him on Twitter @LucasNolan or email him at lnolan@breitbart.com