It says mortgages with arrears of three months and over were 0.95 per cent at the end of April compared with 0.57 per cent in December 2007.
The firm says it has strengthened procedures on mortgage arrears capitalisation. It previously had a policy to let some borrowers have payment holidays of up to six months.
Gross lending in the first quarter plummeted to just £1.2bn, compared with £17bn in the first half of last year.
Rock has announced its first mortgage range since nationalisation, with short-term fixed rates and buy to let mortgages from 6.49 per cent and lifetime mortgages from 6.99 per cent.
Executive chairman Ron Sandler says: “We intend to reduce our mortgage book by accelerating redemptions but need to maintain a presence in the mortgage market and originate a model level of new loans without distorting competition.”