Technology giants such as Google could threaten the stronghold that the biggest accounting firms have in global tax advisory work, making inroads into automating tax returns and other services, according to a new report.

But for now, the Big Four accounting firms—Deloitte LLP, Ernst & Young LLP, KPMG LLP, and PwC, also known as PricewaterhouseCoopers LLP—have leveraged their control of the audit market to grab the tax work and remain dominant, owning 87 percent of the global tax advisory market,...