More Than parent company RSA Insurance warned last month's St Jude's storm would leave it nursing a hit of up to £65 million after it wrought havoc across the UK and northern Europe.
The insurer said wind damage and power cut claims across its UK, Scandinavia and Baltic businesses would impact its bottom line by between £45 million to £65 million.
Britain escaped the worst of the storm, but Scandinavia took the brunt after winds picked up speed over the North Sea, hitting more than 110 mph - the highest ever measured in Denmark.
RSA warned its full-year results would be knocked as the storm added to a weather-beaten year so far for the group after a flooding catastrophe in Canada over the summer also took its toll, with weather losses "materially" worse than expected.
Shares dropped more than 6% as RSA said its return on equity - a key financial measure for the group - would be less than 10% as a result.
RSA chief executive Simon Lee said: "2013 is proving to be an exceptionally tough year for weather events for the group."
He added: " Our priority has been to provide the support our customers need to get back on track as quickly as possible."
The group saw customer calls leap by a third in the UK after the storm, with claims mainly for building damage and power cuts destroying freezers full of food.
But the most extreme conditions over Britain were seen in largely unpopulated areas, which meant there had been "relatively limited" claims in the UK.
Scandinavia suffered as the storm hit heavily populated areas, with an "exceptionally" high level of roof damage reported.
The woes came after a torrid first half for RSA following extreme rainfall and storms in Canada, which caused the country's worst Canadian natural catastrophe on record in June.
This was then followed by further severe flooding in July and ongoing poor weather in the third quarter, leaving it with losses of £83 million.
Full-year figures will also be impacted by a surge in bodily injury claims in the Irish motor insurance market, although it said the financial impact remained unclear at this stage.
Barrie Cornes, insurance analyst at Panmure Gordon, estimated that RSA's full-year earnings would be £55 million lower as a result of the weather warning.