The risk of cyber attacks is rising and slow-burn effects
are additional to short-term costs such as notifying customers, paying ransoms or public relations expenses, the report, written with consultants KPMG and law firm DAC Beachcroft, said.
"There is a lack of understanding as to what cyber attacks can mean," Lloyd's of London Chief Executive Inga Beale told Reuters.
"Businesses need to prepare for the full costs of a cyber attack."
Lloyd's has a 20-25 percent share of the $2.5 billion cyber insurance market, Beale said.
British broadband company TalkTalk (>> TalkTalk Telecom Group) suffered a data breach in 2015 which caused a one-off cost of $52 million, but also led to slow-burn costs of more than $44 million, including an estimate for lost revenue, the report said.
Ransomware attacks are on the increase, the report said, such as the Wannacry attack which infected 300,000 computers in more than 150 countries last month.
A major ransomware attack this week hit computers at Russia's biggest oil company, the country's banks, Ukraine's international airport as well as global shipping firm A.P. Moller-Maersk (>> AP Moller-Maersk).
Cyber crimes where attackers masquerade as chief executives to email finance staff and trigger fraudulent payments are also rising and causing "significant financial losses", the report added.
(Reporting by Carolyn Cohn; Editing by Susan Fenton)