Isobella Thomas's family is one of 2,000 families who say their lives have been ruined by the MMR vaccine.

Her sons Michael, nine, and Terry, seven, developed signs of autism and suffered bowel problems after being inoculated as toddlers.

Mrs Thomas, from Brighton, says Michael became seriously ill hours after his injection and in the following months, became withdrawn, antisocial and affected by stomach cramps.

She took him to her GP regularly but was told his behaviour was not a cause for concern.

It was only when her second son was immunised and reacted the same way that she realised something was wrong.

Mrs Thomas is now a member of the pressure group Jabs which aims to promote understanding about immunisations and offer basic support to any parent whose child has a health problem following vaccination.

The group wants comprehensive information for all parents to make an informed decision on the benefits and risks of vaccination.

A spokesman said: "We accept most children suffer no ill-effect from immunisation but we are concerned very few studies have been done as to the long-term effects of vaccinations.

"What we are looking for is to give the parents the right to weigh up everything about the process and make an informed choice.

"These are not panicking parents, they are people with serious concerns that should not be dismissed."

Vaccines contain weakened or dead germs that cause certain diseases.

To fight these germs, an individual's immune system creates antibodies which help the body fight off the germs and prevent infection.

Some of these antibodies will stay in the body for use at a later time, if needed.

Later in life, if a child or adult is exposed to these diseases, the antibodies multiply and fight them off.

The combined measles, mumps, rubella (MMR) vaccine is given to children or adults to prevent those three diseases and their associated complications.

Measles can cause rash, fever, ear infection, pneumonia, brain damage and, very rarely, death.

Mumps leads to swollen glands, fever and diseases that can damage the brain and nervous system.

One in four adult men may suffer painful swelling of the testicles but sterility rarely occurs. Rare complications include encephalitis and meningitis.

One in 25 children who get mumps will develop deafness and, in some of them, it will be permanent.

Rubella, also known as German measles, can cause a rash, slight fever, aching joints, headaches, discomfort and runny nose.

If a woman gets rubella in the first three months of pregnancy, her baby is at risk of having serious birth defects or dying.

Health experts insist there is no scientific evidence to back claims about MMR's link to autism and say the vaccine is safe.

The MMR jab was initially hailed as a breakthrough when it was launched in October 1988.

Uptake of the vaccine was about 90 per cent until a paper in the medical journal The Lancet alleged a link between it and autism.

A growing number of parents are now calling for single vaccines to be given separately for each disease instead.

Former GP Peter Mansfield, from Lincolnshire, was ordered to appear before the General Medical Council for giving up to 300 children single immunisation jabs which are unlicensed in this country but the case was later thrown out.

Both East Sussex, Brighton and Hove and West Sussex health authorities encourage people to have their children immunised and say the vaccine is safe.

They say the lower the number of children who have the jab, the higher the risk of an epidemic.

The single vaccines available are less effective than those used in the MMR and they are not licensed for use because experiments have shown problems about their safety and effectiveness.

West Sussex has a higher-than-average uptake of the vaccine but parts of East Sussex are lower, especially in the Brighton and Hove area, and fall below the average rate for England, which is about 84 per cent.

The World Health Organisation recommends 95 percent of the population should be immunised to prevent an outbreak.