Stung by a malpractice lawsuit, prominent Miami attorney Dexter Lehtinen has struck back at his former client, the Miccosukee Indian Tribe, by disclosing highly sensitive information about its longstanding tax battle with the federal government.

Contrary to the tribe’s public position that its members don’t have to pay income taxes on gambling profits routinely divvied up on the reservation, Lehtinen says he cautioned the Miccosukees years ago that they could be on the hook for millions of dollars in back taxes.

Lehtinen, a former U.S. attorney in Miami who later provided legal advice to the Miccosukees for almost two decades, has disclosed internal memos that reveal his private advice to the tribe — even as he fought a massive IRS investigation.

On Friday, he took the extraordinary step of revealing the formerly privileged information in a bid to dismiss the malpractice suit filed by the west Miami-Dade tribe in November in state Circuit Court.

In his dismissal motion, Lehtinen says he even advised the tribe to create a tax-reserve fund as a “prudent step” should the Miccosukees have to settle with the IRS. The tribe, under former Chairman Billy Cypress, put more than $20 million in the fund. But after Cypress lost his post to Colley Billie in late 2009, the tribe scrapped the fund and “dissipated” the millions set aside by continuing to dole out unreported cash gifts to tribe members, according to Lehtinen’s motion.

Although the tribe as a sovereign nation doesn’t have to pay taxes, its approximately 600 members could be obligated to pay tens of millions of dollars in personal income taxes on gaming distributions over the past decade. Also, the tribe itself could be liable for failing to report the payments and withhold income taxes.

Calls seeking comment from a Miccosukee spokeswoman and the tribe’s lawyer, Bernardo Roman, were not returned.

In their suit, the Miccosukees accused Lehtinen, whom they fired in 2010, of misleading them and providing bad advice on their huge income-tax problem. The tribe asserts that it paid Lehtinen $50 million for a variety of services starting in 1992, but that following his counsel has landed it in a costly legal battle with the IRS.

In a prior interview, he denied any misconduct, saying: “The basis for their not paying taxes was their position, not mine.” Lehtinen, the husband of U.S. Rep. Ileana Ros-Lehtinen, R-Miami, declined to comment on his dismissal motion.

As a sovereign nation, the Miccosukee Tribe as an entity is not subject to taxes, although individual members must pay taxes on income from the tribe or any other source, including distributions from Indian gaming profits generated by its casino operation off the Tamiami Trail, according to federal law and legal experts.

The Miccosukees have historically kept secret their gambling revenues from the bingo-style slot machines and poker games — estimated to be in the billions of dollars over the past decade — and how they distribute the profits to tribe members. They also have flouted regulatory laws requiring the tribe to file reports with the Bureau of Indian Affairs, officials said.

In his dismissal motion, Lehtinen, represented by Miami attorney Joseph Klock, mainly pursues a technical strategy. He claims the Miccosukees cannot sue him in circuit court for legal advice he gave them on the Indian reservation because it is in the Everglades National Park, where there is no state jurisdiction.