The Food and Drug Administration is cracking down on 23andMe and other companies that sell genetic tests directly to consumers.

The F.D.A. sent letters this week to five companies involved in that business, saying their tests are medical devices that must receive regulatory approval before they can be marketed.

“Premarket review allows for an independent and unbiased assessment of a diagnostic test’s ability to generate test results that can reliably be used to support good health care decisions,” Alberto Gutierrez, who is in charge of diagnostic test regulation at the F.D.A., wrote in the letters.

The letters, posted on the F.D.A. Web site on Friday, say the companies must apply for approval or discuss with the agency why certain test claims do not require such approval.

But the letters stop short of saying the tests must be taken off the market until they are approved. Dr. Gutierrez said in an interview that it would be unfair to remove the tests from the market because the agency had not, until now, clearly told the companies that the devices needed approval.

23andMe and two other recipients of the letters, Navigenics and DeCode Genetics, sell tests that scan a person’s DNA, looking at genetic variations that can suggest whether a person is at a higher or lower risk of getting diseases like cancer or diabetes. The most prominent of the companies, 23andMe, is backed by Google and run by Anne Wojcicki, the wife of Google’s co-founder, Sergey Brin.

Illumina, which also received a letter, sells so-called DNA chips that are used by some companies to do the DNA scans. The fifth recipient, Knome, offers consumers a complete sequence of their DNA, which can be used to glean disease risk information.

The F.D.A. action is the latest salvo in a long-running debate about whether and how such tests should be regulated.

On one side are some doctors, geneticists and state regulators who say the tests should be regulated because the results might be used to make medical decisions. Some also say a doctor should be involved in ordering the tests and interpreting the results.

On the other side are those, especially 23andMe executives, who argue that the services merely provide information, not medical diagnosis, and that consumers have a right to the information contained in their genes. At a time when consumers are taking more control of their health care, denying them such information would be, as one director of 23andMe recently put it, “appallingly paternalistic.”

The companies have also said that their tests do not require F.D.A. approval because they have been developed and are offered by a single laboratory. The F.D.A. has typically refrained from regulating such tests, as opposed to test kits that are widely sold to laboratories, hospitals and doctor’s offices.

The F.D.A. is now clearly deciding in favor of regulation, saying the tests of disease risk can have medical consequences.

“It is not unknown for women to take out their ovaries if they are at high risk of ovarian cancer,” Dr. Gutierrez said. Some of the services are also offering consumers information that could be used to determine the doses they should get of particular drugs, like the blood thinners, warfarin and Plavix.

But Dr. Gutierrez denied that the agency was being paternalist. “We really don’t have any issues with denying people information,” he said. “We just want to make sure the information they are given is correct.”

In a statement Friday, 23andMe said it disagreed with the F.D.A.’s conclusion but was open to discussion on ways to regulate the personal genetics industry. “We are sensitive to the F.D.A.’s concerns, but we believe that people have the right to know as much about their genes and their bodies as they choose.”

By contrast, Kari Stefansson, head of research at DeCode, said the interest by the F.D.A. was “timely, appropriate and welcome” because the company wanted its test to become part of standard medical care. A spokesman for Knome said the company would cooperate with the F.D.A.

The F.D.A. recently sent a similar letter to another company, Pathway Genomics, which had planned to sell its test through Walgreen drugstores and eventually other chains. Walgreen dropped its plans to sell the test, though Pathway still offers it online.

The House Energy and Commerce Committee is also looking into these tests and send letters last month to 23andMe, Navigenics and Pathway Genomics requesting extensive information.

Concern about the tests was also raised this week when 23andMe said that because of a laboratory mix-up, up to 96 customers might have received genetic information belonging to someone else.

Daniel Vorhaus, a lawyer specializing in regulation of genetic testing and author of the blog Genomics Law Report, said some of his clients were holding back introducing tests because it was unclear which would require regulation.

“No one has a clear understanding of where the F.D.A. is drawing the line at this point,” said Mr. Vorhaus, who does not represent any of the five companies that received the letters. He said the F.D.A. was “trying to keep up with a commercial space that is moving way faster than they are capable of.”