Trust faces prosecution after series of preventable errors in operating theatres, including surgery on the wrong body part and swabs being left inside patients

The cases of six patients who were the victims of serious errors at a major teaching hospital are to be used to spotlight the dangers posed across the NHS by surgeons who fail to make basic checks before operating.

In an unprecedented move, the Care Quality Commission, the NHS regulator, last week threatened to prosecute Plymouth Hospitals NHS Trust unless it made immediate improvements. Of the six incidents, four involved surgical swabs left inside patients, one patient had an operation on the wrong part of the body and another had an anaesthetic in the wrong place.

They are among 111 so-called "never events" that occurred across the NHS last year. Never events are defined as incidents that should never happen because they are dangerous and easily avoided.

Next month the CQC will publish the result of its Plymouth investigation, which will carry a warning to other trusts to tighten implementation of the surgical checklist introduced in February 2010. It is the first time the CQC has publicly reprimanded a trust and demanded improvements before publication of its report.

A CQC spokesman said: "We are saying: this is a problem now and you need to fix it immediately. We wanted to speed things up, so we came out with what we had got."

The Department of Health published a list of eight never events last year, with a warning to trusts that they would not be paid when they occurred. Despite this, more than two never events a week were recorded in NHS hospitals in England last year. Over half were related to surgery on the wrong site and a third involved feeding tubes being wrongly inserted into the lungs instead of the stomach.

The average cost of dealing with each error was £35,000, adding up to £3.9m for the 111 never events. The Secretary of State for Health, Andrew Lansley, last week announced a trebling of the never events list, with financial penalties for trusts. "We have identified 25 preventable incidents – 'never events' – which should never happen in a high-quality healthcare service and for which payment can be withheld across the NHS," he said.

There are eight million operations performed in Britain each year and around 20,000 patients die after surgery, but it is not known how many might be preventable.

The surgical checklist requires surgeons to run through a series of basic checks (Is this the right patient? Is this the right limb?) in the same way that pilots check their aircraft. It has been described as the biggest clinical innovation in 30 years, after a study by the World Health Organisation in 2008 showed it cut deaths and complications by more than a third.

However, many surgeons dismissed the checklist, saying they were making the checks already and it was too basic. Some adapted the list but experts warned that consistency was vital and each adaptation moved it further away from guaranteeing safety.

At Plymouth's Derriford hospital, which has 35 operating theatres, CQC inspectors found the checklist was "not being fully completed". "We have told the trust that if it does not demonstrate full and consistent compliance with safety checklists from this point forward, our next steps may include prosecution or closure of services," the CQC statement said.

Dr Alex Mayor, the medical director for the Plymouth trust, said none of the six patients had suffered long-term harm. "These events are unacceptable and we are very disappointed. I would like to publicly apologise to anyone affected by these errors.

"Patient safety is paramount and we are determined to ensure there is full and proper compliance with a single, mandatory checklist immediately."

Case study...

Nicola James, 45, was admitted to Derriford Hospital, Plymouth, for a routine gall bladder operation.

Soon after the surgery on 3 November the mother of two began to deteriorate rapidly. She was in severe pain with a distended abdomen and swollen arms and legs.

After three days she was sent for an X-ray, and the next day, a Sunday, she had an ultrasound scan. Surgeons decided to operate immediately and a retained surgical swab was removed.

Neither Mrs James, the daughter of former Lord Mayor of Plymouth David James, nor her family was told about the swab. They were told her bile duct had been "nicked".

Mrs James developed pneumonia and remained in hospital for a further week, seriously ill. Eleven days after the second operation, her parents Diane and David were told about the swab. "The sister called us in. She said the surgeon had been up to see Nicky that morning and had apologised profusely and told her that a swab had been left in her during the first operation. They said they were very sorry," Mr James said.

"My family were distraught. We thought we were going to lose our daughter. We thought she was going to die, but they were so blasé about it.

"Nicola's case happened in November. There was another case in January. They have learned nothing."

A Plymouth Hospitals NHS Trust spokesman, said: "We are extremely sorry for what happened to Ms James and for the distress it has caused. We are undertaking a full investigation."