Freddie Mac (FRE.P) (FRE.N), the second-largest U.S. home funding company, on Thursday said its mortgage investment portfolio shrank by an annualized 50.9 percent rate in April, while delinquencies on loans it guarantees accelerated.

The portfolio decreased to $830.3 billion for an annualized 9.5 percent increase year to date, the McLean, Virginia-based company said in its monthly volume summary.

In April 2008, the portfolio was $737.5 billion.

The delinquencies that increased stress on the company's capital jumped to 2.44 percent of its book of business in April from 2.29 percent in March and 0.81 percent in April 2008.

Freddie Mac said the temporary suspension of foreclosure transfers, which expired on March 6, contributed to the increase in single-family delinquency rates.

Freddie Mac said refinance-loan purchase volume was $43.3 billion in April, down from March's $52 billion, which was its largest refinance month since 2003.

The company's total mortgage portfolio decreased at a 8.1 percent annualized rate in April to $2.231 trillion.

In early September 2008, the U.S. government seized control of Freddie Mac and its larger sibling, Fannie Mae (FNM.P) (FNM.N). The takeover came as worries increased about shrinking capital at the congressionally chartered companies.

The government is now relying heavily on Fannie Mae and Freddie Mac in its efforts to stimulate the U.S. housing market, which is in the midst of its worst downturn since the Great Depression, by buying more mortgage loans, easing refinancing and helping homeowners avoid foreclosure.

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