As many as a dozen Chicago firefighters are facing disciplinary action for allegedly looking the other way while a colleague had sex with a woman at a South Side firehouse.

The firefighter accused of engaging in the firehouse sex — with a woman who was his girlfriend at the time — is facing either a lengthy suspension or termination, City Hall sources said.

The other firefighters, some of whom have already retired, stand accused of “varying degrees of not paying attention or looking the other way,” sources said. They either knew or should have known that the hanky-panky was going on.

“The Chicago Fire Department does not tolerate any conduct unbecoming on its long tradition of valor and professionalism,” Chicago Fire Department spokesman Larry Langford said in an emailed statement.

“Following the allegations of improper activity in a firehouse, an internal investigation was immediately opened. The investigation was completed on January 17, following the complete review of all evidence and nearly two dozen interviews, and the accused has been served charges. CFD Labor Division is finalizing the charges and will be issuing findings soon.”

Jim Tracy, president of the Chicago Firefighters Union Local 2, could not be reached for comment.

Langford refused to name the accused firefighters or the location of the firehouse, citing strict rules included in the union contract.

Sexual harassment allegations against Hollywood movie mogul Harvey Weinstein have prompted an avalanche of allegations against men in acting, the media, the restaurant industry and politics.

Against that backdrop, the woman’s allegation about sex in a Chicago firehouse had to be taken seriously — and it was.

That’s even though it was only after the firefighter broke off the relationship that the woman chose to file a complaint with the Fire Department’s Bureau of Internal Affairs.

To verify the woman’s claims, Internal Affairs interviewed nearly two dozen firefighters, and went through telephone and text messages provided by the woman.

Firehouse sex is strictly off-limits. In addition, no visitors are allowed in Chicago firehouses after 10 p.m.

Even with those rigid rules, it’s not the first time that salacious charges have been made against Chicago firefighters.

In 2000, a 27-year-old prostitute arrested for burglarizing a suburban home claimed she performed oral sex on as many as five firefighters a night at several Chicago firehouses over 11 years.

At the time, Jennifer Manzella was being held in Cook County Jail on charges that she burglarized the Elmwood Park home of a retired Chicago firefighter with whom she said she had an 11-year relationship.

She told police she was paid $25 for every sex act she performed, that the sex went on at “several” Chicago firehouses and that her roommate was performing similar acts for on-duty firefighters.

James Joyce, then the fire commissioner, threw the book at those accused of having firehouse sex with a prostitute and against firefighters who used their fire engine as a taxicab for Belmont Harbor partygoers who claimed they were too drunk to drive home.

But that wasn’t it for firehouse sex scandals.

In 2004, the Chicago Fire Department cut off Internet access at 59 firehouse computers — and ordered dozens of firefighters to undergo counseling — after determining that a computer at an O’Hare Airport station was used over a nine-month period to visit “inappropriate sites,” possibly including pornography.