A surge of natural disasters around the world meant insurance market Lloyd's of London suffered its most costly six-month period in its 323-year history.
It has announced a pre-tax loss of £697m for the first half of the year compared to a profit of £628m in the same period during 2010.
Between January and June, Lloyd's customers made claims worth £6.7bn, up 24%.
Lloyd's paid out £1.2bn as a result of the earthquake and tsunami in Japan in March.
The earthquake in New Zealand a month earlier triggered claims of £865m - most of these made by the country's government.
At the start of the year flooding in Australia - the worst in decades - led to claims in excess of £200m.
Lloyd's said the predicted claims from Japan would make it the fourth-largest event to hit the market.
The biggest was Hurricane Katrina in the US in 2005, which produced £2.4bn in claims.
Out-going Lloyd's chairman, Lord Levene, said: "The claims seen so far in 2011 arising from major events have already exceeded the total for 2010 and we have not yet reached the end of the Atlantic hurricane season."
He added that historically low investment yields in many classes of business have added to the challenge.
Lloyd's is a society of corporate underwriters and wealthy individuals that make insurance transactions through 88 syndicates.
It is the world's fifth-largest insurer measured by premium income.