Union fat cat Mark Rosenthal spends more time sleeping at his desk than organizing labor, a series of damning photos reveals.

The 400-pound president of Local 983 of District Council 37 — the city’s largest blue-collar municipal-workers union — often downs a huge meal, then drops into dreamland in the early afternoon, members of the union’s executive board told The Post.

“He eats lunch when he arrives at work at 2 p.m. Then, like clockwork, he goes to sleep with a cup of soda on the table and the straw in it,” said Marvin Robbins, a union vice president.

“Then he wakes up, looks at his watch and says, ‘I have to get out before the traffic gets bad.’ He’s usually out by 4 p.m. after being at the office two hours.”

Rosenthal is a former Parks Department employee who rose to power campaigning to rid the union of corruption in the late 1990s.

He last made embarrassing headlines in 2009, when he inspired a City Council bill requiring jumbo-size ambulances for morbidly obese patients after he had a stroke at City Hall.

Since then, he hasn’t been making much of an effort to give the city’s ambulances a break and slim down. Union officials say he racks up $1,400 in monthly food bills on the union dime.

Much of the 5-foot-7, 400-plus-pound Rosenthal’s food tabs are for catered union events and meals he writes off as “union business,” board members claim.

They say he significantly overorders at eateries like Dallas BBQ, the Stage Door Deli and Pine Restaurant in The Bronx, a hangout for local politicians, and takes the extra food back to his Bronxdale apartment.

“He’s always walking off with a doggie bag or extra boxes of food,” said another executive board member.

Rosenthal, who earns $156,000 annually, yesterday denied being a free spender— and insisted he works “12-to-14-hour days.”

He says the allegations are “part of a smear campaign” by a faction trying to get another Local 983 vice president, Joseph Puleo, elected president in a June 5 showdown.

He said it’s normal for executives to take “power naps.”

He also blamed his meetings with the sandman on the effects of pain medication he takes for backaches he has suffered since he fell through a chair at a McDonald’s last year.

“The chair broke because I’m big,” Rosenthal said.

“I’m 60 years old, so if I eat during my lunch hour and take a little medication, can’t I close my eyes?” he said outside his apartment complex. “Is it so outrageous?”

Rosenthal is also under fire from the union’s executive board for allowing lawyer Arthur Schwartz to allegedly rack up an average of $12,000 a month in union legal fees for years despite being on a $5,500 monthly retainer, board members said.

But Schwartz claims he has submitted only one monthly bill over $10,000 in 15 years representing the union and averages about $7,000 per month in fees.

The executive board on May 15 voted to fire Schwartz anyway — and also to pull Rosenthal’s union car.

Board members said they were furious enough to fire Schwartz because he pursued a lawsuit on Rosenthal’s behalf aimed at changing the makeup of the union’s election committee after it nominated Puleo as a candidate for president on May 7.

Rosenthal responded to Schwartz’s firing by filing another suit days later in Manhattan Supreme Court, claiming the May 15 meeting occurred without his approval.

The suit also accuses executive board members of using union resources to sway the election in Puleo’s favor.

“Mr. Puleo and his cohorts have basically seized control without having won the election,” the suit says.

“Not only that, [but] they have [also] assigned legal work to attorneys, including to Mr. Puleo’s campaign lawyer.”

Puleo called Rosenthal’s allegations “absurd,” adding, “He’s the one using the union’s resources to sue members in good standing.”

The case has since been moved to a federal court in Manhattan with a hearing set for today.

The union represents 3,000 workers — mostly Parks Department peace officers and maintenance workers and NYPD tow-truck operators and other traffic agents that are among the lowest-paid city workers.

But they still fork over $1,080 in annual union dues that help fund Rosenthal’s salary and perks.

Rosenthal has headed the union since 1998, when he won an election under the platform of ridding the union of corruption and alleged mob ties. At the time, he called the union a “cesspool.”

Some members say he was a strong labor advocate for the union in his early years, but his questionable spending and sleeping habits — and alleged lack of attention to union issues — in recent years led to Puleo’s campaign.

He has also ruled the union with little opposition in part because he and Schwartz have strong political connections at City Hall, so members say they were afraid to go up against them until now.

“There was always the fear that he’d use his power to retaliate against anyone who spoke up,” Puleo said.

“He always likes to say he’s a big supporter of Mayor Bloomberg and the fact that the mayor called him to thank him for his support when he was elected.

“I would love to see the mayor’s face if he saw the big sodas that he likes to drink. It’s kind of ironic.”

Additional reporting by Lorena Mongelli