A rat gnawed away on a terrified vulnerable patient as he lay sedated in his hospital bed, his parents have claimed.
Jason Ketley, 42, was left with painful injuries after being bitten by the rodent more than a dozen times on his back and neck.
But hospital staff only discovered what was happening when Mr Ketley, who has a mental age of two, was spotted stumbling around a corridor with the rat hanging from his neck by its teeth.
Nurses at the specialist care unit rushed to Mr Ketley's aid and knocked the rat off him before killing it, the Daily Mirror reported today.
Mr Ketley's outraged mother Pat Boardman, 64, told the newspaper: 'I was horrified. He was quite heavily sedated and wasn’t able to defend himself.
'He was completely helpless and terrified. It’s a disgrace. He was very scared and the staff had to show him they had killed the rat to prove it could no longer hurt him.
'I’m appalled that this sort of thing could happen to my son in an NHS hospital in this day and age.'
But hospital bosses have claimed the rat was in fact a field mouse.
On average, most adult rats are about 10 inches long and weigh between 350 to 600 grams, whereas mice are generally around four inches long and weigh just 30 grams.
Mrs Boardman was disgusted at hospital staff's 'outrageous' suggestion her son was attacked by a field mouse because of the size of the bite marks.
Mr Ketley, who suffers from bi-polar disorder - was attacked by the creature shortly after going to bed at a specialist care unit in St Ebba’s hospital, Epsom, Surrey.
But Mrs Boardman, who lives with Mr Ketley’s stepdad Tom, in Bognor Regis, West Sussex, only heard about the incident after calling the unit to see how her son was doing.
Mr Ketley was taken to Epsom General Hospital where he was given injections against diptheria, tetanus and polio before being returned to the unit.
His parents have made a formal complaint to the Surrey and Borders Partnership NHS Foundation Trust about the incident in November.
Jo Young, Director Quality and Nursing at Surrey and Borders Partnership NHS Foundation Trust, said: 'The events described here are genuinely shocking and I would like to reassure Mrs Boardman that we are taking her concerns extremely seriously.
'This is why we have logged the events as a serious incident and are conducting a detailed investigation.
'Our records indicate that a mouse was seen in the house in November and that pest control was alerted immediately but we are unable to provide a complete response to all the concerns raised until we know the outcome of our investigation.
'I would like to stress that the reported incidents are extremely unusual and that we took urgent action to ensure the safety of people using services on the site.
'We take our responsibility for caring for vulnerable people very seriously and we will act on any findings from our investigation to ensure our high standards of care and support are maintained.'