Don't expect a initial public offering of shares of Twitter anytime soon.

CNN is reporting that the company has placed restrictions on employees who hold shares, forcing them to keep 80 percent of the shares they own. The cable news outlet cited company emails about the policy, which has been in effect for a year, and said at least one high-level employee resigned because of the policy.

There has been widespread speculation that Twitter would follow Facebook and float an IPO, perhaps as soon as 2013. But, officially, the company has made no public comment about its plans.

The restriction, according to CNN, is primarily in place because it helps keep Twitter under 500 shareholders. While companies can remain private after they hit that threshold, they are required to publicly disclose certain financial information to the Securities and Exchange Commission. Facebook was reportedly nearing that number, which may have factored in its decision to file an IPO earlier this month.

In August, Twitter's senior technical engineer, Evan Weaver, sent an email to the entire company saying he was resigning over policy differences, according to CNN. Later that same day Twitter CEO Dick Costolo sent out an email explaining the policy and the main reason for it.

"We don't want to be public until we have very predictable quarterly earnings growth," Costolo wrote in his August email. "We're not ready to be a public company for a couple years."

We've asked Twitter for comment and will update if we hear back. The company declined CNN's request for comment.