Breslin collapsed at his hotel in Nice, France, on Wednesday and died at a hospital there, his wife, Carol, said from their apartment in New York. She said she was told doctors believed he died of a heart attack.

His death was first reported by Anne Midgette of The Washington Post, co-author with Breslin of the 2004 book “The King and I,” which detailed Breslin’s work with Pavarotti.

Under Breslin’s guidance, Pavarotti moved beyond opera houses to become an entertainment star who performed at arenas, stadiums and even Las Vegas. The rotund tenor also appeared on television shows and an American Express commercial.

“That I think was the essence of our plan together,” Breslin said during a 2004 interview on radio station WQXR.

Born on Oct. 1, 1924, Breslin became an opera fan as he grew up in New York.

After working in public relations for Chrysler and the Bulova Watch Co., he became an unpaid publicist for the Santa Fe Opera, then started his own company in New York near Carnegie Hall, Herbert Breslin Public Relations. His first three clients were sopranos Elisabeth Schwarzkopf, Joan Sutherland and Marilyn Horne. He also played a big role in the career of pianist Alicia de Larrocha.

Known for boasting about his clients and his sometimes profane language, Breslin started with Pavarotti in 1967, six years after the singer’s professional debut and a year before he first appeared at the Metropolitan Opera. They were introduced by Terence McEwen of London Records when Pavarotti was the cover singer for Carlo Begonzi for a Carnegie Hall performance of Verdi’s Requiem with conductor Herbert von Karajan and the orchestra of Milan’s Teatro alla Scala.

Breslin started as Pavarotti’s publicist and became his manager. They worked together through 2002 – five years before Pavarotti’s death.

Their relationship soured in their later years together, especially after Pavarotti signed with promoter Tibor Rudas for Three Tenors concerts with Placido Domingo and Jose Carreras.

In “The King and I,” Breslin detailed Pavarotti’s behind-the-scenes life, telling tales of gluttony and mistresses and confirming rumors that the tenor couldn’t read music.

“He didn’t like to study at all,” Breslin said during the WQXR interview.

In addition to his wife of 57 years, Breslin is survived by daughter Andrea, son Eric and four grandchildren. His wife said she had not yet determined funeral arrangements.