The Chapter 11 protective bankruptcy filing covers WorldCom and all of its active US subsidiaries, but not its non-US subsidiaries.
Chapter 11 provides a financially beleaguered company a method to keep operating under protection from its creditors while developing a plan for resolving its financial problems. The filing means that WorldCom will not have to pay interest due on loans taken out earlier.
WorldCom will use the court protection to restructure its debt, and sell off non-core assets. The company has debt estimated at $32.8bn (£20.8bn), several thousand corporate customers, serves around 20 million consumers and runs the world's biggest Internet network.
Subsidiaries include Internet infrastructure company UUNet Technologies and telecommunications carrier MCI Communications.
UK companies using WorldCom networks include National Lottery provider Camelot, Barclays, BP, Compaq, Dell, Diageo, EDS, Goldman Sachs, the London Stock Exchange and Toyota.