The largest U.S. mortgage insurer MGIC Investment Corp (MTG.N) said it may not be able to repay amounts owed under its notes due in 2011, and posted a worse-than-expected third-quarter loss due to an increase in delinquencies. The insurer said it expects its books to generate material incurred and paid losses for a number of years and it may not be able to repay senior notes maturing in September 2011.

For the quarter ended Sept. 30, the insurer posted a net loss of $517.8 million, or $4.17 a share, compared with a net loss of $115.4 million, or 93 cents a share, a year earlier.

Total revenue fell more that 10 percent to $413.3 million.

Analysts had expected a loss of $1.62 a share on revenue of $437.0 million, according to Thomson Reuters I/B/E/S.

The insurer said the percentage of loans that were delinquent rose to 13.97 percent from 7.54 percent a year earlier.

Shares of the Milwaukee-based company fell 18 percent to $6.00 in pre-market trade Friday.