Six weeks later, little Harriet, who before the routine jab had seemed so perfect, happy and healthy to her doting parents, died aged 16 months.

Now Mr and Mrs Moore, of Peasedown St John, near Bath, like hundreds of other families across the UK whose children have suffered permanent damage or died after having the vaccination, are determined to find out the truth about its safety.

Virtually overnight after having the jab at the end of March 1996, Harriet went off her food and wanted to be held all the time. Three weeks later she was rushed to the intensive care unit at Bath's Royal United hospital suffering uncontrollable fits.

From there she was flown by air ambulance to Great Ormond Street Children's hospital in London, where powerful drugs were used to control her fits. But she went into a coma-like state and never regained consciousness.

She died in her parents' arms in June 1996 of liver failure and encephalitis after being flown back to the RUH.

Policeman Mr Moore, aged 40, and his wife, 32, are taking their case to a Department of Health Vaccine Damage Tribunal in a bid to establish if the MMR vaccine killed her.

"Her death nearly destroyed us. A nightmare doesn't describe it," Mr Moore said yesterday.

"We are not after compensation, we just want to establish the truth for Harriet's sake. We think other parents should know the full facts before deciding if their children should have the vaccination." Mrs Moore said they were told Harriet might show a few symptoms after the jab.

Mr and Mrs Moore, who have an eight-month-old son, Guy, and are expecting another baby in June, will not give either child the vaccine. A Depart- ment of Health spokeswoman said a booklet issued to parents contained information about all confirmed adverse reactions. Independent scientific evidence did not support a link between MMR and long-term adverse reactions.

"MMR is the safest way for parents to protect their children from diseases which themselves kill or disable. We would still say that on balance the benefits of the MMR outweigh the risks," she said.

Solicitors Dawbarns, of Kings Lynn, Norfolk, have been contacted by more than 600 families, including the Moores, since being appointed to co-ordinate the MMR claims five years ago.