'It is a good rule in life never to apologise,' wrote PG Wodehouse. 'The right sort of people do not want apologies, and the wrong sort take a mean advantage of them.'

Luckily for patients, the NHS has scorned his words. 'When things go wrong, say sorry' is the new official advice being given to doctors and nurses across Britain.

A document outlining the 'Being Open' policy is being sent to all NHS trusts. It gives step-by-step advice on how to deal with patients who have been unintentionally harmed. Guidelines explain how to arrange a meeting, who should be there, how to communicate and what should be discussed. Top of the list is a simple apology.

'Our core ambition is to change the nature of the NHS - to say that when things go wrong we can be open and say sorry,' said Professor Sir John Lilleyman, medical director of the National Patient Safety Agency, who issued the guidance.

Apologising was something NHS staff had been 'hopeless' at, because they feared disciplinary action or litigation. The guidelines stress that 'saying sorry is not an admission of liability'.

Lilleyman said there was an 'ill-founded' but common 'fear that you will be sued to kingdom come. The paradox is that if you apologise, explain it was a genuine error and [promise] to learn from it, the patient will often say they understand and leave it at that.' But when things were covered up, anger pushed some into taking action.

The children's cancer specialist said, for example, that six years ago he realised just in time the wrong blood was being given to a patient.

'It was an error in the labelling. We had not given her enough to do major harm but had to give her lots of antibodies. I spent a nervous afternoon explaining it to her parents. We said we were sorry, we would find the cause and it wouldn't happen again.' As a result the parents did not take it further.

The guidance tells hospitals to put the policy in place by March. The document acknowledges that apologising can be difficult but it lays out a prototype for doing so. Staff are advised to set up a meeting as soon as possible with the patient, ensuring the 'most senior person responsible for the patient's care' is there. 'Communicate with someone as you would a member of your own family', without jargon or acronyms. Do not to attribute blame and follow up the apology in writing, it says.

Charles Vincent, professor of clinical safety research at Imperial College, London, said this was essential to maintain a patient's trust. 'Healthcare staff want to [say sorry] but it is difficult and no one has taught them how to do it.

'If a patient is harmed they may need further support but they lose trust in that health organisation or in medical staff generally and become anxious about any treatment. That can cause huge problems later on.'

But an apology alone was not enough. Although the new guidelines might reduce litigation, patients who deserved compensation should get it, Vincent said.

Peter Walsh, head of the patient group Action against Medical Accidents, said: 'What is scandalous is that in so many cases there is not openness and honesty [now]. This is a step in the right direction but we want to see it given teeth and truly enforced.'