Citigroup Inc. Chief Executive Charles Prince is planning to resign at a board meeting on Sunday, according to people familiar with the situation, as the bank faces big new losses from distressed mortgage assets.

The move would end the four-year tenure of Mr. Prince, a longtime lawyer and loyal lieutenant of former Citigroup head Sanford Weill, who assembled the financial giant that stands as America's largest bank by assets. It would make Mr. Prince the second major chief executive in finance to leave his job in a week, following the ouster of Merrill Lynch & Co.'s Stan O'Neal.

Mr. Prince, 57 years old, moved before the board considered his fate. His tenure has been rocky. He faced pressure to cut costs, and more recently, debt-market turmoil has taken a tremendous toll. Citigroup's stock is down 31% this year and almost 9% in the last week. People familiar with the matter said the Securities and Exchange Commission is looking into the bank's accounting for its off-balance sheet investment funds that have recently attracted scrutiny.

"Chuck took a very unusual move and is resigning," said one person familiar with the situation. "He's stepped up and done the right thing without forcing the board to act."

Just a few weeks ago, board members including Robert Rubin, the influential chairman of Citigroup's executive committee, expressed support for Mr. Prince and said that his job wasn't in jeopardy. "I think Chuck's going to be here for a lot of years," Mr. Rubin said in an interview last month. A spokeswoman for Citigroup declined to comment Friday.