There were more than 100 escapes from psychiatric hospitals in England and Wales last year, it emerged today.

According to figures released to the BBC, at least 116 patients in medium and low security psychiatric hospitals escaped from the institutions or from their escorting staff.

Four patients remain at large. That compares to just five escapes from prisons and their escorting staff in 2007.

The number of escapes from hospitals amounted to 23 times more than from prisons.

The investigation comes in the wake of the case of Darren Harkin who raped a schoolgirl after absconding from a psychiatric unit.

The 21-year-old had also been allowed to build a vast collection of horror and pornographic DVDs while being detained for repeatedly stabbing his six-month-old step-brother to death in his cot.

Reading Crown Court heard how he escaped from the low-security Hayes Hospital, near Bristol, and brutally raped a 14-year-old girl.

Yesterday, Harkin, 21, was made the subject of a hospital order under the Mental Health Act and detained indefinitely at maximum security Broadmoor Hospital.

Former chief inspector of prisons, Sir David Ramsbotham said today's figures were "horrifying".

He told BBC Radio 4's Today programme: "It is a horrifying figure of course, but it's not one that surprises me because the medium and low secure units in the NHS do not have same degree of security a prison does."

He added: "The wake-up call is actually to the Government to build more facilities inside prisons because the public will know that those prisoners who have been sentenced for a crime and have mental health problems are being properly looked after inside what is essentially a secure place."

Professor Louis Appleby, national director for mental health in England, told the programme there had to be a "mature debate" in society about the appropriate care mentally ill people received.

It was an "inevitable consequence" some patients could escape from units as part of the "balance" needed between risk to the public and a "humane mental care system".

The incident involving Harkin was "appalling" and there was an inquiry under way.

He said: "It is important to understand what the figures mean.

"Obviously the incident which was in the news yesterday was an appalling incident and it quite clearly shouldn't have happened.

"There will be some very serious questions about the care that man received and the treatment decisions that were made and there is an inquiry going on which will report at the end of this month."

"On the broader issue... it is very important to understand what low-secure is for from a clinical point of view.

"The person in a low-secure unit is a person with a long-term low-grade illness such as schizophrenia. They are not a person who is a highly dangerous individual."

Professor Appleby said comparing escapes with those from prisons was not a "reasonable comparison".

"There is a difference here between a prison and a secure unit - a secure unit is a hospital - the people who are in it are patients and they are ill," he added.

"If you give the impression that there are 100 people are getting out and roaming the streets and threatening the public that is not an accurate impression about the risks that the mentally ill people present to the public."

He went on: "A prison removes people from society, a secure unit prepares people to return to society - its purpose is treatment and rehabilitation.

"If it's a humane society, we want people to return to normal life then we will have to accept the clinicians will make decisions which balance the risk people present and the care they need to receive to help them live a normal life."

He added: "We have to accept that a humane mental care system prepares people for life outside hospital and if some low-risk people are occasionally out of the hospital when they shouldn't be, that may be part of, it is not acceptable, but it is perhaps an inevitable consequence of getting that balance right."