FRANKFURT (Reuters) - The finance chief of Deutsche Boerse <DB1Gn.DE> warned on Wednesday rivals in the United States and China would become dominant if the German group's planned merger with the London Stock Exchange <LSE.L> was blocked by European regulators.

Gregor Pottmeyer said the planned $28 billion tie-up, announced earlier this year, was necessary after competitors in those regions had cast their eyes over the European exchanges in the past.

"Otherwise the Americans will sit down together with the Chinese to regulate the market," he said at an industry event in Frankfurt.

European antitrust regulators, concerned the merger could hinder competition in key financial market activities, have opened a full investigation into the deal and the European Commission is expected to detail its objections in the second week of December.

U.S. group Intercontinental Exchange <ICE.N> had been considering a counter-offer for the LSE but ultimately dropped out of the race.

A failure of the European exchanges merger could lead to "the Intercontinental Exchange knocking on London's door again," Pottmeyer said.

America's second-biggest exchange, the Chicago Mercantile Exchange <CME.O>, might then consider a takeover of Deutsche Boerse, he added.

(Reporting by Andreas Kroener; Editing by Tina Bellon and Mark Potter)