FRANKFURT - Deutsche Bank expects the cost of a major overhaul in the works to be up to 5 billion euros ($3.4-$5.6 billion), one person familiar with the matter said on Wednesday.

CEO Christian Sewing flagged an extensive restructuring in May. He promised shareholders "tough cutbacks" to the investment bank to turn the lender around after it failed to agree a merger with rival Commerzbank.

The lender, Germany's largest, is planning on cutting between 15,000 and 20,000 jobs, or more than one in five of its 91,500 employees.

A spokesman for Deutsche declined to comment on the expected cost of the restructuring. The bank said it was working on measures to accelerate its transformation so as to improve its sustainable profitability.

"We will update all stakeholders if and when required," the bank said.

($1 = 0.8863 euros)

(Reporting by Tom Sims; Editing by Elaine Hardcastle) ((Tom.Sims@thomsonreuters.com; +49 69 7565 1242; Reuters Messaging: tom.sims.thomsonreuters.com@thomsonreuters.net))