More must be done to protect NHS staff from violence and abuse, the National Audit Office has said.

In the last two years the number of reported incidents of violence and aggression against NHS staff working in acute, mental health and ambulance trusts has increased by 13 per cent.

While better awareness of the need for reporting violence and aggression has contributed to the increase, many trusts say that increased hospital activity and higher patient expectations have also contributed to the growing problem.

The NAO estimates that the direct cost is likely to be at least £69 million a year.

This excludes staff replacement costs and the human costs, such as stress, low morale, lost productivity and high staff turnover, which "are known to be substantial".

The watchdog also estimates that almost two in five incidents involving doctors and consultants go unreported.

It says that more needs to be done to ensure that staff understand how and why they should report incidents.

And it adds that the NHS needs to do more to establish partnerships with the local police, the Crown Prosecution Service, social services and the media.

The NAO said that all groups must "ensure that there is a clear understanding of different organisations' roles leading to a clear and consistent approach to dealing with violent individuals and incidents in NHS settings".

"Effective staff training is crucial in tackling violence and aggression," added the report.

NAO chief Sir John Bourn said it was "unacceptable" for health care workers to be subject to violence and aggression on a daily basis.

"Apart from the immediate impact on the individuals concerned, the experience or threat of violence causes increased stress and sickness absence, lowers staff morale and drives individuals out of the health sector at a time of serious staff shortages.

"Good progress has been made through the zero tolerance zone campaign, but the NHS needs to demonstrate clear improvement across trusts in incident follow up, staff training and partnerships with other public agencies," Sir John said.

Liberal Democrat spokesman Dr Evan Harris said ministers had failed to tackle the causes of violence.

"More work must be done, particularly with homeless people, to reduce inappropriate visits to Casualty," he said.

"Of course staff must be supported in seeking prosecution. There should also be work on preventing violence through more secure designs for accident and emergency departments, and secure environments for GPs to treat persistently violent patients."