Aon failed in its bid to buy Newman Martin and Buchan last week, market sources said.
It is understood that the deal fell through.
Newman Martin and Buchan, which trades in the Lloyd's and Company markets, specialises in energy, marine and non-marine insurance and reinsurance.
It employs a large number of specialist ex-Aon staff, making it an attractive acquisition for Aon.
A Newman Martin spokesman said it was not company policy to comment.
An Aon spokesman said that the company did not comment on individual acquisitions.
Newman Martin and Buchan was founded in 1987, led by Gordon Newman, Chris Martin and Barry Buchan.