Screenshot by Stephen Shankland/CNET

Google has begun showing advertisements in the space reserved for e-mail messages in Gmail users' inboxes, a new step in the company's effort to turn its free services into revenue sources.

The company has shown ads alongside Gmail messages for years, but these ads appear as messages that can be opened like e-mails and forwarded to others, according to Gmail users who started seeing them last week. They appear only in the new "promotions" tab of Gmail's new multi-tab interface, and they're marked with a pale yellow background and labeled "ad," similar to how Google treats some search ads that appear above or to the right of search results.

"These ads are part of the Promotions tab in the new inbox in Gmail," Google said in a statement. "If you disable the Promotions tab or prefer another inbox style, you will see the old style of ad that runs across the top of your inbox."

The ads aren't likely to sit well with a lot of people already dealing with plenty of inbox clutter -- especially those who have appreciated Gmail's abilities to weed out spam. "Inbox tabs should only include your messages, not ads disguised as email," opined Alex Chitu on the unofficial Google Operating System blog after seeing the ads last week.

So why risk user wrath? Money -- and the belief that the new ads are an improvement. Google had this to say about the business rationale for the move:

As always, advertising keeps Google and Gmail free to use. We work hard to make ads safe, unobtrusive, and relevant. Instead of ads always appearing at the top of your inbox, they've been relegated to a more appropriate place in your Promotions tab to create a better overall experience.

Google also said the new ads are more relevant than earlier Gmail ads. They replace the old-style ads above the inbox or to the right of messages unless people disable the Promotions tab.

The ads can't be marked as spam, but closing them will make a particular ad go away until you refresh the browser or wait a long time, Google said. In addition, people can use Gmail's ads preferences manager to block specific advertisers.

The ads are apparently part of the Gmail Sponsored Promotions program that's in limited beta, according to Marketing Land, which said Google charges advertisers when people click on the ads.

Google works to keep search ads relevant to the people who see them, and doubtless will try to take that approach here, too. They'd better, given how much more prominent they are.

So far, Google still isn't showing ads in Google+.

Update, at 2:37 p.m. PT: Adds Google comment.

(Via CNET Australia)