The U.S. Food and Drug Administration has issued a regulatory warning to the company at the center of the federal investigation into Virginia Gov. Robert F. McDonnell, alleging that Star Scientific has been improperly selling the dietary supplement backed by the governor and his wife.

In the letter sent to the company Dec. 20 and posted on the FDA’s Web site Tuesday, the agency says Star made claims about its supplement, Anatabloc, that would require the product to receive approval as a drug.

The FDA also accused the company of unlawfully selling products containing anatabine, the key chemical found in Anatabloc and another Star product, CigRx, without proper notification to the FDA.

The conclusion was based on a review of Star’s Web site and marketing materials.

Star officials have called Anatabloc a nutritional supplement with anti-inflammatory properties that does not require FDA approval because its key chemical is found in food crops such as eggplant and potatoes.

The FDA warning gives the company 15 days to address the allegations or face potential further action. In a statement on its Web site, Star said it is responding to the FDA and “has already advised the agency that it intends to work cooperatively to resolve these issues.”

Federal prosecutors informed McDonnell and his wife, Maureen, last month that they intended to charge the couple in connection with illegally promoting Star in exchange for gifts and loans from its then-chief executive Jonnie R. Williams Sr.

That decision was put on hold after the McDonnells’ attorneys made a face-to-face appeal to top Justice Department officials in Washington. A final decision about whether to charge the couple is now not expected until this month.

In August 2011, the McDonnells hosted a luncheon at the governor’s mansion to mark the introduction of Anatabloc to the market. A company news release at the time indicated that the company presented research into the product for doctors and other health-care professionals at the event.

McDonnell has apologized for his relationship with Williams but insisted he did nothing to assist the company beyond what he would do for any state-based company. The governor says he made no promises to Williams in exchange for his largesse, which included more than $165,000 in gifts and money to McDonnell and his family.

The FDA action raises new questions about why the McDonnells became entangled with a tiny dietary supplement company that had been losing money for years.

In addition to hosting the mansion event, which McDonnell has said was intended to recognize Star for giving research grants to Virginia universities, the governor and first lady set up meetings for Star officials with top state health officials.

Maureen McDonnell also attended Star meetings out of state. At one event in Florida, she told a group of investors that she believed Anatabloc could be used to lower health-care costs in Virginia.

A picture of McDonnell holding up a bottle of Anatabloc and smiling was, for a time, featured on the product’s Facebook page. His spokesman has said that while the governor has used Anatabloc, the photo was unauthorized. It was taken down shortly after The Washington Post first published details of McDonnell’s relationship with the executive.

Williams is cooperating with federal authorities. People familiar with the investigation have said he has indicated that he believed McDonnell was helping the company.

Jason Miyares, a spokesman for McDonnell’s legal team, declined to comment on the FDA letter. In the past, however, he has said that Williams’s account of his relationship with McDonnell cannot be trusted, citing a history of trouble that Williams’s past business ventures have had with government entities.

Miyares has also alleged that Williams falsified expense accounts dealing with his interactions with McDonnell.

The FDA action could provide McDonnell’s legal defense, should one become necessary, with new evidence to bolster claims that Williams and his company are not credible.

In its letter, the FDA cites a series of Star news releases about research conducted on anatabine showing that the synthetic chemical, found naturally in tobacco and other plants, might have beneficial impacts on Alzheimer’s disease, traumatic brain injuries, multiple sclerosis and other conditions.

“Your product Anatabloc is not generally recognized as safe and effective for the above referenced uses and, therefore, this product is a ‘new drug’ ” under the law, the FDA concluded. “New drugs may not be legally introduced or delivered for introduction into interstate commerce without prior approval from the FDA.”

Pieter Cohen, an assistant professor at Harvard Medical School who studies dietary supplements, said the FDA’s letter to Star is especially strong, breaking new ground for the industry.

In the letter, the FDA said that just because a chemical like anatabine is present in foods does not mean it can be sold without approval as a dietary supplement.

Instead, any chemical that wasn’t marketed as a dietary item prior to the adoption of new supplement laws in 1994 must be considered a “new ingredient,” the FDA indicated, and a company must by law send notification of its sale as a supplement to the FDA.

That has not been the widespread practice of the nutritional supplement industry, Cohen said. Since 1994, there have been fewer than 170 such notifications acknowledged by the FDA even as the number of supplements on the market has ballooned from 4,000 to an estimated 85,000, Cohen said.

“They’re making some clear basic points in this, mainly about the fact that just because something is found in tomatoes and potatoes doesn’t mean you can pharmaceutically produce it or extract it in large quantities and sell it as a supplement,” said Cohen, who is also an internist at the Cambridge Health Alliance.

He predicted that the company will have to discontinue the product’s sale as a supplement and instead explore whether it wishes to seek approval for the chemical as a new drug, a costly proposition.

In addition to the McDonnell investigation, Star faced a federal securities probe, although it has informed investors that it expects to face no charges as a result of that investigation. It is also being sued by shareholders who claim the company overstated Anatabloc’s promise.

Star has been attempting to move on from the Virginia experience. Williams stepped down as chief executive last week, and the company has indicated that it is contemplating changing its name to Rock Creek Pharmaceuticals.

Star also announced Tuesday that Williams, who remains a major company stockholder, has extended the company a $15 million line of credit to help fund operations.