LOS ANGELES (REUTERS) - The accountancy firm behind the biggest blunder in Oscars history is staying in the picture.

The Academy of Motion Picture Arts and Sciences said on Wednesday it has decided to retain the services of PricewaterhouseCoopers (PwC) despite a backstage envelope mix-up that led to the wrong film being announced winner of the best picture Oscar last month.

"After a thorough review, including an extensive presentation of revised protocols and ambitious controls, the Board has decided to continue working with PwC," Academy president Cheryl Boone Isaacs wrote on Wednesday (March 29) in a letter to Academy members.

Isaacs added that the Academy had been "unsparing in our assessment that the mistake made by representatives of the firm was unacceptable."

The error led to musical La La Land being declared the winner and its producers and cast celebrated and started acceptance speeches on the stage before Moonlight was named the real winner of the night's top prize.

PwC, which has overseen Oscar balloting for 83 years, removed the two accountants responsible from further involvement in the Academy Awards and carried out an investigation into tightening up its procedures.

Starting next year, the Academy said PwC will place a third accountant in the Oscars show control room, who will be able immediately to notify the director should a mistake be made.

All accountants will also have to hand over their phones and other electronic devices before going backstage, Isaacs said.

That decision followed news that one of the accountants involved in the Feb 26 blunder had been taking photos backstage with celebrities and posting them on Twitter during the ceremony.