The Fire Department and its uniformed firefighters union are embroiled in a free-speech dispute over locker decoration.

The union says the department’s policy, which bars “controversial” material from being posted on firefighters’ lockers, also appears to restrict its members from putting up American flags and religious cards. In several cases, union leaders say, captains have peeled flags off lockers.

But a deputy fire commissioner, Mylan Denerstein, said the dispute is “just a disingenuous issue” and said the union was told last week that religious cards and flags are allowed.

The union’s lawyer, Ronald Kuby, speculated that an offensive locker posting might have triggered the crackdown. Still, he called the city’s policy unconstitutional.

“We entrust New York City firefighters with our lives and the lives of those most precious to us, and we don’t trust them to properly decorate a locker?” Mr. Kuby said.

In a letter sent Tuesday to Fire Commissioner Nicholas Scoppetta, Mr. Kuby called the policy “problematic, paternalistic, and demoralizing.” He threatened to sue on First Amendment grounds if the city doesn’t repeal it.

Ms. Denerstein said the department is trying to comply with antidiscrimination laws.