In a blog posting by David Drummond, Google's chief legal officer, says it is notifying the other companies and is working with U.S. authorities.
Google also says it has evidence that the Gmail accounts of human rights activists were also accessed, although it does not think contents of e-mail were compromised.
Google does not specifically say who was behind the breaches.
"These attacks and the surveillance they have uncovered -- combined with the attempts over the past year to further limit free speech on the web -- have led us to conclude that we should review the feasibility of our business operations in China," wrote Drummond.
Drummond continued, "We have decided we are no longer willing to continue censoring our results on Google.cn, and so over the next few weeks we will be discussing with the Chinese government the basis on which we could operate an unfiltered search engine within the law, if at al. We recognize that this may well mean having to shut down Google.cn and potentially our offices in China."