The Financial Conduct Authority has place five City aviation insurance brokers under investigation over alleged misuse of client information.
The watchdog seized computers from five firms, including Aon, JLT, Marsh and Willis Towers Watson, according to a report in the Insurance Insider.
The regulator is said to be investigating accusations that the firms in question mishandled client information, with sources claiming the FCA is trying to determine whether confidential information was passed from brokers to underwriters.
Aon said aviation broking accounted for less than $100m (£78m) in global revenue last year and added that it is "working diligently with the FCA in order to ensure it can carry out its work as efficiently as possible".
"We cannot comment on the details of this investigation as it is ongoing, other than to say Aon takes compliance and regulatory issues extremely seriously and has a strong and respectful relationship with the regulator," the broker added.
JLT said in a statement that it "is participating in this investigation and confirms that JLT Specialty is providing all appropriate assistance to the FCA".
"We are unable to comment on the ongoing investigation beyond saying that JLT takes all regulatory matters very seriously and has a constructive relationship with all the regulators that it works with across the JLT Group," the company said.
Willis Towers Watson also confirmed that the FCA recently initiated an investigation "into certain business practices".
The group added: "The focus of the investigation relates to and is limited to aviation broking. Willis Towers Watson is cooperating fully with the FCA in relation to its investigation."
Marsh said: "In connection with a civil competition investigation that the Financial Conduct Authority is undertaking into the aviation insurance and reinsurance sector, the FCA recently conducted an on-site inspection at Marsh Limited's office in London."
The broker said the FCA "indicated that it had reasonable grounds for suspecting that Marsh Limited and others have been sharing competitively sensitive information within the aviation (re)insurance sector".
"Marsh is cooperating fully with the FCA's ongoing investigation and is conducting its own review with the assistance of outside counsel," the firm added.
The FCA declined to comment.