New York City Mayor Bill de Blasio on Friday defended his right to maintain the confidentiality of his correspondence with five close friends who serve as outside advisers, calling it a matter of “principle.”

The firms that employ all but one the friends got hundreds of thousands of dollars in payouts from a mayoral nonprofit that accepted contributions from donors with business before the city.

“It’s a general principle ... I just think if you you’ve got a standard that’s set, again, with legal guidance — we stick to that standard,” de Blasio told WNYC’s Brian Lehrer on their weekly “#AskTheMayor” radio show.

De Blasio and his lawyers have deemed the five friends “agents of the city” — a designation the administration says exempts their correspondence from open-records laws.

He said he discloses correspondence when the exchanges concern the agents’ private clients.

De Blasio also reacted to a poll that showed his job performance rating had sunk to his lowest ever.

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The Quinnipiac University survey, released Tuesday, followed news reports over the past two months that said de Blasio, his inner circle and a now-defunct nonprofit he set up to push his agenda are under at least five probes examining their fundraising practices.

“There’s relentless negative headlines,” he said Friday.

The poll found that 41 percent of voters approved of how de Blasio is doing his job, and 52 percent disapproved.