Greg Case, CEO of London-based Aon P.L.C., said Monday that the United Kingdom's insurance market would be disadvantaged if the country votes to leave the European Union.

In a statement Monday, Mr. Case said foreign investment, trade and supply chains could be constrained if the United Kingdom votes to leave the European Union on Thursday.

And in a separate statement Monday, the British Insurance Brokers' Association asked its members to carefully consider the merits of the United Kingdom remaining part of the European Union saying it “unequivocally” believes that the British brokerage sector will be better off if the country votes to remain a part of the European Union in the upcoming referendum.

Mr. Case said there were several reasons why he believed the London insurance market would be severely challenged if the United Kingdom leaves the European Union.

“First, it is absolutely vital for our business to attract and retain the best and brightest talent from around the world,” he said.

This would be challenged if Britain voted to leave, Mr. Case said.

In addition, many of the most challenging international insurance programs are placed in London, he said, and Aon believed that insurance buyers “could potentially lose coverage options in a post-Brexit world.”

“As an example, U.K.-based insurers may be disadvantaged by their lack of ability to easily do business with the European Union, a freedom they currently enjoy, and a general environment of uncertainty regarding solvency requirements after a leave vote,” he said.

In addition, competing insurance centers in Europe, such as Frankfurt and Munich in Germany or Paris, might draw business away from London, Mr. Case said. And the supply chains of British multinational companies may be weakened “as formalized links are replaced with a structure that has yet to be determined.”

Mr. Case said that, as an organization, Aon had made significant investments in the U.K., including shifting its global headquarters to London in 2012.

“Our firm took these steps with confidence because we believe in the U.K. market, believe in the power of connectivity over that of isolation, and believe in what we can accomplish as a firm with superior talent, markets, and innovation,” he said.

London-based BIBA said in a statement Monday that it believed there were several reasons why leaving the E.U. would put U.K. brokers at a disadvantage.

Among other things, it said that the current system of “passporting” whereby insurers can write business throughout the European Union on the basis of being regulated in their home state, could stop, potentially resulting in a reduction in choice for buyers.

“I am firmly in the 'remain camp',” said David Hunt, chairman of BIBA, in the statement.