Bad loans at the retail mortgage operations of General Motors, the world's biggest carmaker, more than doubled last year as it increased lending to poorer customers and was forced by post-Enron accounting to keep more debt on the balance sheet.

Non-performing loans in the mortgage business of General Motors Acceptance Corp, the finance arm that is the most profitable part of the carmaker, increased from $1.3bn to $3.4bn in 2004, excluding purchases of distressed debt. As a result, the proportion of the loan portfolio that is more than 60 days overdue rose to 8.8 per cent from 5.2 per cent the previous year, according to regulatory filings submitted this month. The company has not previously disclosed non-performing loans.