A veteran New York City firefighter fell 20 feet while preparing for a training exercise in his Brooklyn station house — and was left with such serious injuries, he will not survive, officials said Friday.

William P. Moon, who has served on the department for 21 years, was being kept on life support, as doctors prepare to use his organs for transplants, Mayor Eric Adams said at a press conference.

“This morning, it became clear that he will not survive his injuries,” the mayor said. “His family has made a difficult decision to donate his organs to save the lives of others.”

The 47-year-old was getting ready for a drill at Rescue Company 2 on Sterling Place near Howard Avenue in Ocean Hill just before noon Monday when he fell and suffered a serious head injury, the FDNY said.

Sources said that the rescue company was about to practice saving workers trapped on window washing equipment, and he fell from a platform used to simulate such emergencies.

“This type of work requires extensive training,” he FDNY Chief of Department John J. Hodgens. “Billy was preparing for one of these training exercises when this tragic accident occurred.”

His fellow firefighters treated him at the scene, before he was rushed to the Kings County Hospital Center in critical condition. 

Adams hailed Moon’s decision to be an organ donor.

“Firefighter Moon is a hero twice over,” the mayor said. “He spent 21 years saving the lives of his fellow New Yorkers, and he will continue to save lives in his passing.”

Rescue Company 2 is an elite unit that “responds to any type of emergency you can imagine happening in the city of New York,” FDNY Chief of Department John J. Hodgens told reporters. 

In addition to helping trapped window washers, the unit also focuses on helping construction workers trapped by collapses and drownings, Hodgens said.  

Moon was appointed to the FDNY on May 5, 2002, the department said. 

He began his career at Ladder Company 133 in Queens, where he worked for 20 years before the department reassigned him to Rescue Company 2.

“His actions [in Queens] – it’s a busy ladder company no doubt – contributed to the saving of hundreds of lives throughout his 20-year career,” Hodgens said. “Billy was a type of firefighter who wanted to do more,” Hodgens said. 

Moon was also a member of the Islip Volunteer Fire Department on Long Island and served as their Chief of Department in 2017.

An Islip resident, Moon is survived by his wife, Kristina, and their two children.

“Loving Billy was not only a marriage of families but a marriage to the fire department,” his wife said in a statement read by FDNY Commissioner Laura Kavanagh. “His love and passion for the job extended far from the firehouse walls. He instilled that same passion in our children and always talked about how important it was to help others. We are eternally grateful for the Christmas miracles that Billy will now be able to give others. Thank you.”

Kavanagh remembered Moon as a dedicated firefighter. 

“Our hearts are broken for Firefighter Moon’s family and friends, and for our entire Department,” Kavanagh said. “He spent his life helping others, here at the FDNY, and on Long Island, as a volunteer Fire Chief. Firefighting was in his bones, and he embodied the spirit and passion of our Department. He was passionate about organ donation, and just as he saved countless lives working out of the firehouse, he will continue to save lives in his passing.”