The move has raised questions about its privacy implications, after similar moves with Gmail and its then-new Google Buzz social network in 2010 led to a row over alleged privacy invasion. Those in turn led to Google being bound to a 20-year privacy oversight by the US Federal Trade Commission.
Google has also made the change opt-out, so that users will have to change their settings to prevent unknown people emailing them. The senders will not see the email address of the person they are sending the message to unless the recipient replies.
Announcing the move in a blogpost, Google product manager David Nachum wrote:
Have you ever started typing an email to someone only to realize halfway through the draft that you haven't actually exchanged email addresses? If you are nodding your head 'yes' and already have a Google+ profile, then you’re in luck, because now it's easier for people using Gmail and Google+ to connect over email.
Marc Rotenberg, the executive director of non-profit Electronic Privacy Information Center, told Reuters that the new feature was "troubling" and added: "There is a strong echo of the Google Buzz snafu".
Buzz created an uproar because it tried to create a social network built out from the email contacts that people had. One woman who had separated from her abusive ex-husband said that it revealed the identity of her new boyfriend to him, potentially endangering her and him. Google's executive chairman Eric Schmidt later said "nobody was harmed" by the moves.
Google says that Google+, set up in June 2011, has 540m "active" users, but has been vague about how it counts activity. Analysts have suggested that Google+ is not a social network aiming to compete with Facebook, but instead a system for collecting more information about people's web use. The number of "active" users will have increased since Google made it obligatory in November 2013 to use Google+ to leave a comment on YouTube.
Google has recently faced criticism for over-tight integration of Google+ into products after one transgender user of an early version of its newest version of Android discovered that Google+ had been integrated into its chat system, and sent a message to somone under the woman's name they were adopting rather than the man's name the intended recipient was used to. The woman had not expected the system to search Google+ for a contact name - but it did.
Facebook also allows people to send messages through a name search, but does not reveal any information such as emails if the person replies.
Google says it will be rolling out the system over the next few weeks and will automatically email all Gmail users telling them of the changes. It is not possible to create a Gmail account without having a Google+ account.
guardian.co.uk © Guardian News and Media Limited 2010