Mayor Bill de Blasio has ordered the Department of City Planning to study mandatory permitting for all new hotels across the city, an unprecedented move that would give the politically powerful Hotel Trades Council tremendous leverage.

The move comes as de Blasio banks on that union’s support for his quixotic presidential bid, igniting a new round of criticism from good-government groups over pay-to-play behavior from a City Hall badly tarnished by a string of campaign finance scandals.

“At a minimum, it creates the appearance of impropriety and illegality,” said Susan Lerner, the head of good government group Common Cause. “The mayor keeps setting up situations that beg for a pay-to-play interpretation.”

Despite the appearances, de Blasio denied any impropriety.

“That’s just inaccurate in every way,” he told NY1’s Errol Louis Monday night.

Special permitting would require any developer seeking to build a new hotel to go through an exhaustive review process, including community board recommendations and City Council approval, before they could start construction, even if the land is already zoned for commercial and hotel uses.

Only hotels opened in recently rezoned neighborhoods like Midtown East or in manufacturing areas are currently covered by such a requirement.

“This is a political decision by the mayor, not a land-use decision,” said Kenneth Fisher, a former Brooklyn councilman and land-use expert.

“That doesn’t mean that it’s a quid pro quo,” he added. “[De Blasio] believes that union membership for hotel workers is a path to the middle class and their political and ideological interests line up on this issue.”

City Planning officials confirmed Monday that the administration requested the study based on conversations that began around April — just a month before de Blasio launched his flailing presidential campaign.

They added there is no firm deadline for the study to be finished.

Crain’s New York Business first reported the news.

De Blasio narrowly avoided state and federal indictment in 2017 for trading favors with donors to a nonprofit controlled by his allies, the Campaign for One New York.

And campaign finance watchdogs have filed complaints with the Federal Elections Commission over de Blasio’s use of loosely regulated political groups to help pay for his White House run.

“He keeps taking the wrong lessons from these reprimands – that he can get away with things, rather than stop doing them,” Lerner added. “It’s deeply upsetting and completely objectionable.”

City Hall offered a different spin on the turn of events.

“This is not new — it’s been in discussion for years,” said de Blasio spokeswoman Jane Meyer. “There’s nothing more to it.”