Just Mayo actually isn’t mayo, after all. At least not according to the Food and Drug Administration.

Federal regulators have warned Hampton Creek Foods, the company behind the vegan-friendly, egg-less spread that has put a dent in Big Mayo’s market share in recent years, that it is violating the law by “misbranding” some of its products.

“The use of the term ‘mayo’ in the product names and the image of an egg may be misleading to consumers because it may lead them to believe that the products are the standardized food, mayonnaise,” the FDA wrote in a letter to the company dated Aug. 12 but posted online Tuesday.

“The use of the term ‘Just’ together with ‘Mayo’ reinforces the impression that the products are real mayonnaise by suggesting that they are ‘all mayonnaise’ or ‘nothing but’ mayonnaise,” the agency continued. “However, your Just Mayo and Just Mayo Sriracha do not meet the definition of the standard for mayonnaise. According to the labels for these products, neither product contains eggs. Additionally, the products contain additional ingredients that are not permitted by the standard of identity for mayonnaise, such as modified food starch.”

The problem, in other words, is not that Hampton Foods is offering an alternative to traditional mayonnaise, but that it is selling it under the guise that it is, at its core, indiscernible from the conventional option.

"It's one thing to enjoy some of the halo for mayonnaise, but it's another to dupe consumers," said Parke Wilde, who is an associate professor at the Friedman School of Nutrition Science and Policy. "I think they're probably a little over that line with 'Just Mayo.' I can definitely see how it's a bit misleading."

Ivan Wasserman, an attorney at Manatt Phelps & Phillips who advises companies on food labeling, marketing issues and compliance with FDA regulations, agrees. The warning letter, he says, isn't all that surprising. "What's surprising is that it took this long. As far as having to comply with the standards of identity, I'd say the FDA has a very strong position,"

Hampton Foods’ chief executive, Josh Tetrick, could not immediately be reached for comment. Tetrick, who has lined up big-name investors such as Microsoft founder Bill Gates and tech entrepreneur Peter Thiel, has long said the San Francisco-based startup’s goal is to produce food that is both healthy and affordable.

Last year he defended his product's name and branding, insisting that the goal was to reach a broad audience.

“We don’t market our product to tree-hugging liberals in San Francisco, even though I’m in the middle of nine of them right now,” Tetrick told the Post. “We built the company to try to really penetrate the places where better-for-you food hasn’t gone before, and that means right in the condiment aisle of Walmart.”

And Hampton Foods has had success in that department, striking deals with a range of huge retailers, including Costco.

But the company’s venture into the market for America’s best-selling condiment, mayonnaise, has been at turns successful and fraught.

Last year, food giant Unilever, maker of popular Hellmann’s mayo, sued Hampton Creek, arguing the company was misleading consumers because Just Mayo was not “exactly, precisely, only and simply mayonnaise.” At least not in the way mayo is defined by the FDA, which says the condiment must include “egg yolk-containing ingredients.”

Of course, as Post reporter Drew Harwell noted in a story about the lawsuit last year, that definition is more than half a century old:

The FDA’s definition of mayonnaise was set in 1957, decades before the phrase “vegan mayo” ever made sense. (Maintaining that “standard of identity” is important: Kraft Foods’ Miracle Whip, which doesn’t meet the FDA’s standard, is technically a salad dressing).

Unilever doesn’t only slam Just Mayo for what it calls confusing branding — advertisements have called the stuff “mayo,” and its logo resembles an egg — it also says the company has no proof in its claims of beating Hellmann’s in a taste test.

Unilever faced some criticism over its lawsuit from consumers and advocates who saw it as a response to the fact that Hampton Creek and other small food start-ups were chipping away at the company’s huge market share.

There is, however, a sense that Hampton Foods has gone out of its way to fit in with the mayonnaise crowd. The name 'Just Mayo,' in some ways, can be seen as an admission on behalf of the company that consumers aren't enthused enough about an egg-less mayonnaise to buy it for that reason alone. Otherwise, why bury the fact that the product is made without eggs? Especially given that that very characteristic is the pillar of Hampton Foods' mission.

"The marketing really does beg a lot of questions," Wasserman said. "There's the imagery of the egg, and the lack of other indicators. A product with that name should probably inform customers of what the product is and what it is not in other ways."

Unless, of course, the hope is that consumers learn that the product is made without eggs only after having eaten it, and, ideally, enjoyed it.

"I take alternatives to animal based foods very seriously as a solution to a sustainable food system," Wilde said. "But I don't think anyone should be duped by a mayo product that might be different from what they think it is."

Last December, Unilever withdrew its lawsuit “so that Hampton Food can address its label directly with industry groups and appropriate regulatory authorities,” Mike Faherty, vice president for foods at Unilever’s North American unit, said in a statement at the time. “We believe Hampton Creek will take the appropriate steps in labeling its products going forward.”

The FDA has also said the company’s cholesterol-free claim on Just Mayo isn’t legitimate because the food contains too much fat (“more than 13 grams of total fat per 50 grams”) to make such an assertion. Hampton Creek has 15 days to respond to the FDA’s recent warning letter.