UPDATE 1-Comcast lowers cable revenue growth forecast

(Adds share movement, details on forecast)

SAN FRANCISCO, Dec 4 (Reuters) - Comcast Corp CMCSA.OCMCSK.O lowered its forecast for 2007 cable revenue growth to about 11 percent from at least 12 percent on Tuesday, citing a "challenging economic and competitive environment," sending the shares down.

Comcast, the largest U.S. cable television operator, also said 2007 cable capital expenditures would be about $6 billion, 5 percent above its earlier expectations.

Comcast attributed the higher expenditures to increased purchases of advanced digital set-top boxes, the company’s accelerated shift to digital technology, network improvements and acquisition-related investments.

Comcast shares slumped 4.7 percent in extended trading following the disclosure, after closing down 32 cents, or 1.5 percent at $20.73 on Nasdaq.

The company gave the revised outlook before a presentation at a UBS AG investor conference on Wednesday.

Philadelphia-based Comcast said it now expects so-called revenue generating units, or the sum of basic and digital cable, high-speed Internet and net phone subscribers, to increase by about 6 million to 57 million. Comcast earlier had said it expected about 6.5 million additions.

Comcast said it was reaffirming all other guidance for 2007. (Reporting by Philipp Gollner; Editing by Jeffrey Benkoe)