(Reuters) – Specialist insurer Beazley PLC is bracing for claims linked to the novel coronavirus pandemic to double to $340 million, with almost all the estimated increase arising from events cancelled in the wake of a recent spike in infections.

Shares in the London-based insurer, whose business is heavily weighted towards the U.S. and U.K., had slumped 15% in their worst one-day fall on record by 0707 GMT.

"With the largest segment being conferences, our clients are still largely unable to operate as restrictions on holding events persist," Beazley said in a trading statement on Tuesday.

The forecast claims rise, which is net of reinsurance, offsets what the firm described as improving growth prospects across its wider business portfolio, primarily driven by higher insurance premium rates, with an overall rate rise of 13% at end-August.

Beazley boss Andrew Horton also blamed the hike in the claims estimate on exhaustion in the reinsurance market, in which insurers look to hive off a portion of their exposures to other insurers.

"We estimated some return to normality in September," Mr. Horton told Reuters, adding that he didn't expect conferences to resume until the second quarter of next year.

The Lloyd's of London insurer said that if this prediction proves inaccurate, claims could rise by a further $50 million net of reinsurance.

"The magnitude of the increase is somewhat surprising and we believe reflects an exhaustion of the reinsurance cover," analysts at JP Morgan said, as it forecast a $123 million fall in full-year pretax profits to a $55 million loss.

Beazley said it had considered the recent Financial Conduct Authority judgement on business interruption wording and did not expect the outcome to have a material impact on its figures.

Fellow Lloyd's of London insurer Hiscox said in April it expected to pay up to $175 million to settle claims arising from travel, events and mass gatherings cancelled in the wake of COVID-19, if disruption from the pandemic lasted more than six months.