Through a system of EU "passports", companies in one EU country can carry out business in all the others while opening only branch offices and reporting only to their home regulator.

"This arrangement is much more favourable than that for insurers based outside the EU," the International Underwriting Association (IUA) said in an emailed statement.

"Companies are not ... obliged to maintain expensive capital holdings in each of the EU member states in which they are doing business."

The IUA represents insurers and reinsurers who operate in London but are not part of the Lloyd's of London market.

"If the UK were to exit, insurers and reinsurers from ... countries such as the United States and Japan might be obliged to choose other European centres over London in order to passport into the EU."

Including Lloyd's of London, the London insurance market had total income of 50 billion pounds in 2013, the IUA said.

Lloyd's of London Chief Executive Inga Beale told a conference this week that Brexit would be "bad for business".

Gerry Grimstone, chairman of Edinburgh-headquartered insurer Standard Life and of TheCityUK, which promotes Britain as a financial centre, said in March that a British exit from the EU would be "disastrous for London and the UK".

($1 = 0.6550 pounds)

(Reporting by Carolyn Cohn; Editing by David Goodman)