** Shares of insurance companies fall in premarket trading, as estimates of industry losses from the worst wildfires in Los Angeles history move higher

** Los Angeles-based Mercury General MCY.N slumps 29.7%, while U.S.-listed shares of Chubb CB.BN down 3.6%

** Insurers AIG AIG.N, Lemonade LMND.N, Progressive PGR.N, Travelers TRV.N fall 3.8%, 3.6%, 4.9% and 3.6% respectively

** Reinsurers Arch Capital Group ACGL.O and RenaissanceRe RNR.N down 1% and 2%, respectively

** J.P.Morgan says overall industry losses expected to be high as expectations of economic losses stemming from the fires have more than doubled since it first broke out

** Brokerage estimates the insured losses from the event could exceed $20 bln and even more if the fires are not controlled, compared with its prior estimate of about $10 bln losses

** ALL, TRV and CB are the most exposed to the California homeowners' market, JPM says, adding that reinsurers ACGL and RNR are exposed as well

** Majority of the insured losses likely to occur in the homeowners' line and primary insurers likely to face a greater burden than reinsurers - JPM

** Reinsurers in recent years have increasingly shifted the risk of catastrophe losses toward primary insurers to reduce their loss burden

(Reporting by Arasu Kannagi Basil in Bengaluru)

((ArasuKannagi.Basil@thomsonreuters.com;))