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In an unusual move, Google is responding directly to former TechCrunch editor Michael Arrington’s allegation that it accessed his Gmail account to root out his source for a major story.

“A few years ago, I’m nearly certain that Google accessed my Gmail account after I broke a major story about Google,” wrote Arrington on his personal blog Uncrunched. “I certainly freaked out when this happened, but I never said anything about it because I didn’t want people to be afraid to share information with TechCrunch. But I became much more careful to make sure that communications with sources never occurred over services owned by the companies involved in the story.”

Google denies that charge.

“Mike makes a serious allegation here — that Google opened email messages in his Gmail account to investigate a leak,” Kent Walker, Google general counsel, said in a statement. “While our terms of service might legally permit such access, we have never done this and it’s hard for me to imagine circumstances where we would investigate a leak in that way.”

The allegation of snooping on user email accounts is especially sensitive for Google. Among other concerns and complaints, the company is currently facing a lawsuit over data-mining student emails.

Arrington’s allegation came shortly after Microsoft admitted to accessing a blogger’s Hotmail account to investigate suspicions that an employee leaked pre-release Windows software via email.

It turns out that Microsoft’s terms of service allows it to access user communication in order to “protect the rights or property of Microsoft or our customers.”

Moreover, the terms of service of Google and Yahoo’s free, hosted email services afford it similar privileges.

Arrington wrote late last week: “I have first hand knowledge of this. A few years ago, I’m nearly certain that Google accessed my Gmail account after I broke a major story about Google.”

The proof, he said, was that a former Google employee — who was his source and was apparently drunk at a party — approached Arrington and said he or she had been “shown an email that proved that they were the source” by people at Google investigating the leak. This email was sent from a non-Google email account to Arrington’s Gmail account, he said.

“The source had corresponded with me from a non Google email account, so the only way Google saw it was by accessing my Gmail account,” wrote Arrington. “A little while after that my source was no longer employed by Google.”

Arrington declined to comment.