BRUSSELS (Reuters) - Aon AON.N has sought EU antitrust approval for its $30 billion bid for Willis Towers WLTW.O, with the EU watchdog setting a Dec. 21 deadline for its decision, a filing on the European Commission website showed on Monday. The deal to create the world"s biggest insurance broker with more pricing power could draw intense regulatory scrutiny, according to analysts. The companies are the second and third largest brokers globally, after leader Marsh & McLennan Companies Inc MMC.N. The Commission can either clear the deal with or without concessions after its preliminary review or it can open a four-month long investigation if it has serious concerns. Aon dropped an earlier bid last year for Willis after media reports broke the news.