Two construction workers were seriously hurt when a gas tank exploded at a 37-story landmark office building in Midtown early Wednesday, city officials said.

The workers were using a Sawzall cutting tool to open a box storing an acetylene tank on the 34th floor of 550 Madison Avenue — when sparks from the blade ignited the blast at around 6:30 a.m., according to the city Department of Buildings.

Hazmat units were called to the historic building — located near East 56th Street, about a block from Trump Tower – to secure the tank, and the leak was shut down, the FDNY said. 

One worker was taken to Weill Cornell Medical Center and the other to Bellevue Hospital. Both were listed in serious but stable condition, according to the fire and buildings departments. 

A third worker was also injured but refused medical attention, according to the FDNY. 

A DOB inspection found the gas tank was being stored in a gang box that wasn’t properly secured and that leaked — causing to the explosion, officials said.

The department determined that the contractor had failed to comply with FDNY code regulations. Inspectors issued “relevant violations,” in addition to a partial stop work order for any “hot work” at the location — meaning any operations that produce flames or sparks.

The DOB also found that an elevator being used for construction operations had an expired permit, prompting a separate partial stop work order, officials said.

The skyscraper, completed in 1984, has previously served as the headquarters for AT&T, and later Sony. It was designated a landmark by the New York City Landmarks Preservation Commission in 2018.

Late last year, the building re-opened its block-long public arcade located between East 55th and 56th Streets, which had been closed for several years.

Just two weeks ago, private investment firm Clayton Dubilier & Rice announced it was leaving the Seagram Building for more than twice as much space at 550 Madison Avenue, which was redesigned by the Olayan Group.

In what appears to be the year’s largest new Manhattan office lease to date, CD&R signed for 144,000 square feet on the 18th-22nd and 26th floors.