US authorities are seeking to block Aon’s acquisition of insurance broking rival Willis Towers Watson — both of which have large marine portfolios — over fears about competition.

The US Justice Department on Wednesday filed a civil antitrust lawsuit to block the $30bn proposed consolidation of what it described as two of the "big three" global insurance brokers.

As alleged in the complaint filed in the US District Court for the District of Columbia, the merger threatens to “eliminate competition, raise prices, and reduce innovation for American businesses, employers, and unions that rely on these important services”.