An attorney for Mayor Bill de Blasio’s now-defunct nonprofit created to push his agenda says the group will stop cooperating with a state ethics probe, accusing investigators of masking “true motivations” and conducting an “unprecedented fishing expedition.”

In a 12-page letter dated Friday, campaign-finance lawyer Laurence Laufer told the executive director of the state Joint Commission on Public Ethics that de Blasio’s nonprofit, the Campaign for One New York, has “grave concerns” about the commission’s “unfortunate actions” and “numerous unexplained anomalies.”

“We will no longer cooperate with what has obviously become a blatantly political exercise by an agency whose very independence is deeply in question,” Laufer wrote.

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The addressee of the letter, executive director Seth Agata, confirmed late Friday that his commission received the letter.

“Lobbyists can’t choose who regulates their conduct,” Agata said. “The law decided JCOPE regulates that conduct, and that’s exactly what we’re doing.”

The commission is investigating whether the Campaign for One New York, which was run by close allies of the mayor’s and took money from donors doing business with the city, ran afoul of state lobbying rules.

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The campaign — which announced in March that it would shut down — supported de Blasio’s efforts to enact his progressive goals, such as tens of thousands of units of below-market-rate housing and free prekindergarten classes for all city 4-year-olds.

Word of the commission’s inquiry came in 2015 — about 10 months before news that the de Blasio administration, his inner circle or both were being investigated by state and federal prosecutors examining his fundraising practices. There are now at least five investigative tentacles of probes of the mayor, his donors and their causes.

Among Laufer’s accusations: the commission lacked an “evidentiary predicate” of lobbying by the campaign but is demanding it prove it hadn’t lobbied; the commission leaked news of its inquiry; and it advanced “obscure and meritless indirect lobbying theories.”

Earlier Friday, during a segment on WNYC’S Brian Lehrer show in which the mayor discussed the probes of his administration, de Blasio said, “I want to continue to cooperate with any of these investigations every way we can.”

Laufer’s letter said the team would continue to cooperate with state and federal prosecutors.