The Competition and Markets Authority (CMA) has suggested that a lack of rivals could lead to a spike in prices for products such as flights, electronics and insurance, whilst also warning against a "lack of transparency" in the way the companies operate.
CMA chief executive Andrea Coscelli said: "Most of us visit social media sites and search on the internet every day, but how these firms work can be a mystery.
"At the end of the study, we'll present our findings to the new government as they decide whether and how to regulate what is an increasingly central sector in all our lives."
Earlier this month, meanwhile, Google founders Larry Page and Sergey Brin announced plans to step down from their roles.
The pair have revealed that they will relinquish their roles as Alphabet's chief executive officer and president respectively - but they will remain on the board of Google's parent company, while Google's CEO Sundar Pichai will become Alphabet's new CEO.
They wrote in a letter to shareholders: "With Alphabet now well-established, and Google and the Other Bets operating effectively as independent companies, it's the natural time to simplify our management structure.
"We've never been ones to hold on to management roles when we think there's a better way to run the company."