The number of student loans held by subprime borrowers is growing, and more of those loans are souring, the latest signs that a weak job market and rising debt loads are squeezing recent graduates.

In all, 33% of all subprime student loans in repayment were 90 days or more past due in March 2012, up from 24% in 2007, according to a Wednesday report by TransUnion LLC.

Meanwhile, the Chicago-based credit bureau found that 33% of the almost $900 billion in outstanding student loans was held by subprime, or the riskiest, borrowers as of March 2012, up from 31% in 2007.

"If you become subprime, it's more likely that you will not pay your debt," said TransUnion Vice President Ezra Becker, who oversaw the study.

The high debt loads could weigh on consumer spending and the economy, said Cristian de Ritis, a senior director with Moody's Analytics, a unit of Moody's Corp. If the defaults continue to increase, "the taxpayer is going to be on the hook for losses," he added.