Richie William, from Catford, south east London, died at Great Ormond Street Children's Hospital in July 1997 after alleged mistakes in the way chemotherapy treatment was given.

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

An internal report by the hospital found "several failures in practice".

Among its recommendations was a request to alter the packaging of drugs like vincristine and to include better warnings that it can be fatal if administered wrongly.

Dr Michael Lee and Dr Dermot Murphy are due to appear for trial at the Old Bailey in January.

But the Crown Prosecution Service told Richie's mother on Tuesday that the case against the doctors was not to be pursued unless further evidence is presented.

The incident took place at Great Ormond Street

Riche's mother Dolores William said: "I feel very let down. When I was first told that Richie was dying I was actually going to go to the police station myself and report it as a crime.

"Eighteen months up the line and no-one is going to answer for it."

Worldwide there have been more than 100 incidents linked to vincristine

Neither the hospital nor the crown prosecution service was willing to comment about why the case was dropped.