D.C. Mayor Muriel E. Bowser is raising doubts over strict new regulations governing Airbnb and other short-term-rental companies, saying the limits recently passed by the D.C. Council are probably unconstitutional and likely to be overturned by the courts.

Bowser (D) issued the warning Tuesday in a letter to the D.C. Council in which she returned the bill unsigned and without a veto.

Normally, that would mean the legislation becomes law after the standard congressional review period. If it goes into effect Oct. 1, the legislation will prevent D.C. property owners from renting out second homes on a short-term basis and bar them from renting spare rooms or basements in their primary residences for more than 90 days a year when they are away.

Supporters of the bill, who say it will prevent Airbnb and similar home-sharing services from gobbling up affordable housing, were relieved that the mayor did not veto it.

But Bowser also said D.C. Attorney General Karl A. Racine has advised that the legislation is “unlikely to survive a potential legal challenge” in light of a Jan. 3 federal court ruling in New York involving similar restrictions on home sharing.

In the New York case, a U.S. district judge blocked a law on the grounds that it went too far in requiring the short-term-rental companies to file monthly disclosures to New York City with detailed information about listings including the identities and addresses of hosts.

Such mandatory reporting would violate Fourth Amendment protections against unreasonable searches and seizures, the judge ruled.

The D.C. law has similar reporting requirements, and it was not immediately clear whether the potential problem could be solved by softening them. That is because the District may need the detailed information about short-term rentals to ensure that property owners comply with the 90-day limit and the ban on using second homes for short-term rental.

D.C. Council Chairman Phil Mendelson (D) said he has asked the council’s general counsel to look at the bill to see whether changes are necessary.

“If we have to make adjustments to the law, we will,” Mendelson said. “I’m not interested in seeing our law delayed.”

The debate over short-term rentals has become part of a broader controversy over gentrification in the District. Critics say short-term rentals are driving up housing costs and giving residential communities an unwelcome commercial feel.

The legislation had the strong support of the hotel industry and its unions, which do not like competition from Airbnb, HomeAway, VRBO and similar companies.

The home-sharing services and many hosts counter that residents need the extra income from renting out their properties to afford to stay in an increasingly expensive city.

Bowser echoed the latter position in addition to raising the legal objection. She said the bill could have “struck a better balance” by protecting homeowners who use short-term rentals “to help make mortgage payments and remain in the District.”

Mendelson expressed annoyance with Bowser for declining to sign the bill, which the council approved unanimously, and for what he said was her failure to voice her concerns earlier.

“I’m unaware of any objections she raised while the measure was pending before the council,” Mendelson said.

Bowser’s letter said the New York City bill “was found to violate the Fourth Amendment because its mandatory reporting requirements amounted to an unconstitutional seizure of booking services’ data.”

Citing Racine’s advice, she said: “The mandatory reporting requirements [in the D.C. bill] mirror those in the New York City bill and would likely fail to pass constitutional muster on the same grounds.”

A spokesman for Racine declined to comment, saying the attorney general’s advice to the mayor is confidential.

It’s Time D.C., a coalition of housing activists and others who support the legislation, said that federal courts in California have ruled in favor of restrictions on short-term rentals and predicted that the District bill would stand.

The group praised “this vital legislation that will protect housing options for permanent city residents and safeguard our neighborhoods.”

Airbnb, whose lawsuit led to the New York ruling, issued a cautiously worded statement that did not address the D.C. bill’s legality.

“We are grateful that Mayor Bowser recognizes the value of home sharing both to the District’s economy as well as to the local teachers, seniors and families who earned $96 million from sharing their home in 2018 and continue to do so to make ends meet,” Airbnb spokeswoman Liz DeBold Fusco said.

“We hope to work with the D.C. Council to address the issues that the mayor highlighted, and provide a true path forward for home sharing in D.C.,” she said.

Fenit Nirappil contributed to this report.