LONDON—A record level of catastrophe claims this year pushed Lloyd's of London into a marketwide loss for the first half.

Lloyd's last week posted a £697 million ($1.1 billion) first-half loss, compared with a profit of £628 million ($991.5 million) for the comparable period last year.

The loss compares favorably with the prediction last week from accountancy firm Mazars Group that Lloyd's likely would post a £1.5 billion ($2.37 billion) loss for the first half.

Despite the loss, Lloyd's is “well-positioned” to handle the tough conditions for underwriters, says CEO Richard Ward, who notes that Lloyd's has net assets of £57 billion ($89.99 billion).

Mr. Ward said the result was “not surprising” given the high volume of natural catastrophe claims that already rank 2011 as Lloyd's third-costliest year ever for catastrophes, after 2001 and 2005.

Lloyd's said its combined ratio for the first half of 2011 was 113.3% compared with 98.7% for the first half of 2010.

Natural catastrophes cost Lloyd's about £2.8 billion ($4.42 billion) in the first half of 2010, said Mr. Ward. Total claims in the first half were £6.7 billion ($10.58 billion), he said.

Tornadoes in the United States caused the market about $600 million in claims, he said.

Losses from flooding in Australia, and the earthquakes in New Zealand and Japan cost the market about $3.8 billion, Mr. Ward said.

Other large losses included about $250 million from the February loss of Maersk Oil's Gryphon North Sea oil and gas installation, which is estimated to have caused industrywide losses of about $1 billion, said Luke Savage, director of finance, risk management and operations at Lloyd's.

Mr. Ward said that while rates were increasing for catastrophe-exposed business, other lines of business, particularly casualty lines, still were softening in some cases, partly because of the amount of surplus capital in the industry.

He said catastrophe excess-of-loss rates had increased by about 100%, while some international catastrophe rates had increased by about 50%, and U.S. catastrophe-exposed business was seeing rate increases of about 5% to 10%.

The market posted investment income of £548 million ($865.2 million) for the first half of 2011 compared with £597 million ($942.5 million) for the first half of 2010.

Lloyd's said its gross written premiums for the first half of 2011 were £13.53 billion ($21.36 billion) compared with £13.50 billion ($21.31 billion) for the first six months of 2010.

This slight increase largely was caused by several Lloyd's businesses transferring books of business they previously had underwritten outside the market into Lloyd's, said Mr. Savage.

Lloyd's, with support from the Assn. of British Insurers, is lobbying the U.K. insurance regulator—the Financial Services Authority—to be allowed to use Solvency II beginning Jan. 1, 2013, despite the likely delay to full implementation of the new risk-based capital rules.