LONDON—Chaucer Holdings P.L.C. on Thursday said its gross written premiums rose 23.9% to £650.3 million ($1.09 billion) for the first nine months of 2009, but warned that problems in its political risk business will dampen its overall results.

The London-based insurer reported net investment income of £46 million ($76.8 million) for the period, compared with a £27.7 million ($46.2 million) loss for the first nine months of 2008.

Ewen Gilmour, Chaucer’s chief executive officer, said in a statement that he was pleased with the strong performance, “although the overall result will be held back by the cautious stance we have adopted in strengthening trade credit reserves within our political risk account.”

Chaucer said it was strengthening reserves by £23.5 million ($39.2 million) to account for potential losses in its political risk trade credit business. After a profitable decade, “this has been a challenging year for the trade credit market, with the global economic downturn continuing to cause payment defaults in developing countries, including Bahrain, Kazakhstan and Ukraine,” the insurer said in the statement.

Chaucer is the latest to acknowledge such concerns, following a recent statement by Beazley P.L.C., another Lloyd’s of London insurer, that it was seeing an increase in political risk losses.

Beazley said it had set aside £33 million ($55.1 million) in reserves for political risk claims, expecting high claims frequency to continue into 2010.