The National Audit Office has been asked to investigate whether a £500m underspend by the NHS was caused by political chicanery at the Department of Health.

Norman Lamb, the Liberal Democrat health spokesman, called in parliament's spending watchdog yesterday after the record surplus was disclosed by the Guardian in an analysis of strategic health authority board papers. They showed NHS trusts responsible for hospitals, mental health, primary care and ambulance services ended the financial year in March with £456.8m in spare cash that could have been used to provide extra healthcare. The total did not include a surplus of £75m forecast by NHS foundation trusts.

The underspend angered health union leaders, who said it was generated by an unnecessarily harsh squeeze on spending during the winter when many NHS trusts economised by closing wards, axing jobs and delaying operations until the start of the new financial year in April.

Mr Lamb said patients and staff bore the brunt of measures designed to save the political career of Patricia Hewitt, the health secretary. She had threatened to resign if the NHS made a deficit in 2006/7 after overspending by £547m in 2005/6.

Mr Lamb said: "Everything was driven by her desire at all costs ... to avoid another deficit. That distorted everyone's judgment and as a result they over-compensated ... [The NAO] should establish whether decisions were rational or distorted by manoeuvring to save [her] political skin."