Omnicom will lay off more than 4,000 employees and fold several well-known advertising agency brands after its $13.5 billion acquisition of rival Interpublic Group, The Financial Times reported Monday, citing an interview with company executives.

Creative agency DDB, founded in 1949, and creative marketing agency MullenLowe will be integrated into Omnicom's TBWA, according to the report.

The company did not immediately respond to a Reuters request for comment.

FCB, one of the largest global ad agency networks owned by IPG with roots dating back to 1873, will be absorbed into Omnicom's BBDO, the report said.

Omnicom boss John Wren said more than 4,000 jobs would be cut as part of the IPG integration, mainly in administrative roles but some leadership positions too, the Financial Times reported.

Reuters could not independently verify the report.