The number of mistakes involving NHS patients being given the wrong medication may total 860,850, according to the National Patient Safety Agency.

It said that in 2005, 36,335 incidents were reported, rising to 64,678 in 2006 and 86,085 in 2007. But Professor David Cousins, a senior pharmacist at the NPSA, said only 10 per cent of incidents were actually reported.

According to reports, 100 of the incidents resulted in serious harm or death, although 96 per cent caused low or no harm.

NPSA chief executive Martin Fletcher said the apparent rise in the number of cases reflected a more open reporting culture and a willingness by NHS staff to report errors.

NHS medical director Sir Bruce Keogh said: “We expect all NHS organisations to examine the NPSA’s recommendations carefully and where necessary take steps to implement them in order to ensure that the services they provide are as safe as possible.”

The figures relate to England and Wales and are taken from reports filed by NHS staff in hospital trusts, mental health trusts and in primary care.