Dr Michael John Lee, a specialist registrar in paediatric anaesthetics, and Dr Dermot Murphy, a registrar in haematology, were found not guilty at a 45-minute formal hearing at the Old Bailey on Tuesday.

The BBC's Stephen Cape: "Several failures of practice occured"

The Crown Prosecution Service took the decision after a fresh evaluation of the evidence in the case of Richie William, from Catford, south-east London, who died at Great Ormond Street Children's Hospital in July 1997.

In October 1998, an inquest was told that Richie died two days after a chemotherapy drug vincristine was wrongly injected into his spine rather than a vein.

A 37-page internal report by the hospital found "several failures in practice". Dr Lee and Dr Murphy, who were involved in the procedure, were subsequently charged with manslaughter.

Great Ormond Street: 37-page report

The two doctors were due to appear for trial at the Old Bailey on Tuesday, but instead the case against the pair was dropped.

The CPS said that Richie had died as a result of a "chapter of accidents and misunderstandings" rather than gross negligence.

First, Richie was unable to take a general anaesthetic because he had eaten a biscuit in the six hours prior to surgery.

Then, he was moved to a general ward because the specialist cancer ward at the hospital was full.

A nurse on the general ward had sent vincristine down to the operating theatre unaware that it should never be used during an operation. The drug's packaging clearly states it is potentially fatal when used incorrectly.

And Dr Lee had injected the drug after taking telephone advice from Dr Murphy, who thought that his colleague was referring to the use of a different drug, routinely used in such operations.

A key expert prosecution witness, Professor Alan Aitkenhead had first stated that he believed Dr Lee's conduct over the injection had fallen "significantly below that which could be demanded of him".

But he later changed his mind and decided that significant failures in the hospital system had greater responsibility for the boy's death than he first thought.

Dr Lee: Not to blame for the death

Speaking outside the Old Bailey Dr Lee said: "The last 18 months have been very difficult for me and those close to me. I am very pleased that it has been accepted by the prosecution and by the court that there is no case against me."

CPS spokesman Brandon Longdon said there was no option but to drop the case.

"In this case there was a significant change of opinion by one of the crown's principal medical experts, as a result of which the crown re-reviewed the case and it was decided there was no longer a realistic prospect of conviction."

Richie's mother, Dolores William said in a statement issued through her London solicitors that the medical profession should be more accountable for its mistakes.

The statement said: "We are still devastated by the way Richie died. He suffered a great deal of pain in the days before his death because of the negligent treatment he received.

"Instead of helping him recover from his illness, Richie was injected with a drug which killed him. We expect hospitals to help cure our children, not give them treatment which kills."

Pauline Chandler, solicitor at Thompsons which is representing Mrs William, said the CPS decision decision would not affect the civil claim for damages that is being pursued in respect of Richie's death.

In a statement Great Ormond Street Hospital Trust said it accepted liability for the case, and that investigations were continuing.