The US Federal Trades Commission is expanding its antitrust investigation of Google to include the search engine’s social network, Google+.

The moves comes after Google introduced a new feature designed to make searches more personalised last week. Dubbed Search plus Your World, the new feature includes the addition of Google+ pages, profiles and posts in search results. However, the new features attracted much criticism, including from rival social network Twitter.

The micro-blogging site said it believed the changes to Google’s search are “bad for people, publishers, news organisations and Twitter users.”

“Twitter has emerged as a vital source of this real-time information, with more than 100 million users sending 250 million Tweets every day on virtually every topic,” Twitter added.

“As we’ve seen time and time again, news breaks first on Twitter; as a result, Twitter accounts and Tweets are often the most relevant results.”

Furthermore, according to Reuters in a letter dated January 12, the Electronic Privacy Information Center urged the FTC to investigate Google in regards to the changes and also raised concerns about the company’s “business practices”.

The FTC’s antitrust investigation into Google began in June last year, after a number of web firms claimed Google's procedure of placing its own services in its 'natural' search results, means it can unfairly dominant the market.

A Google spokesman told Reuters the FTC had not asked about Google+ or indicated it was looking into the new social network as of this week.

"We believe that our improvements to search will benefit consumers by better surfacing social content, and the great thing about the openness of the Internet is that if users don't like our service they can easily switch to another site," Google said.

Meanwhile, the FTC declined comment on the expansion of the investigation.