(Adds European Commission, Liberty Global declined to comment)
BRUSSELS, April 25 (Reuters) - U.S. cable group Liberty Global's bid for Dutch peer Ziggo is likely to face lengthy scrutiny by European Union antitrust regulators because of the complex issues involved, two people familiar with the matter said on Friday.
The extensive probe by the European Commission comes amid growing concerns among regulators and consumers over cable companies' growing clout over broadband and their role as gatekeeper of Internet, television and communications services.
U.S. cable companies Comcast Corp and Time Warner Cable Inc $45.2-billion deal has stirred the ire of public interest groups because of their power over what Americans can watch on television.
Liberty Global, controlled by billionaire John Malone, is seeking to expand its cable empire in Europe, where there is demand for faster Internet and digital television. Its bid values Ziggo and its debt at around 10 billion euros ($13.84 billion.
"The European Commission is expected to open a phase two investigation," said one of the people who declined to be named because the EU competition authority's decision is not yet public.
The Commission is conducting a preliminary review of the case until May 8. After that, it will announce the deeper investigation. A phase two probe can take up to 105 working days.
Commission spokesman for competition policy Antoine Colombani and Liberty Global spokesman Marcus Smith declined to comment.
Liberty Global, which has so far not offered any concessions, may choose to do so during the extensive investigation.
Liberty markets its cable offers under the UPC and Unity Media brands in Germany, Belgium, the Netherlands, and Austria among other countries on the continent and under the Virgin Media brand in Britain.
($1 = 0.7227 Euros)
(Reporting by Foo Yun Chee; editing by Barbara Lewis and Jane Merriman)