Mrs Thomas claims both her sons became ill after having the MMR jab

Isabella Thomas, of Somerton, Somerset, claims the health of Michael, 13, and Terry, 12, deteriorated after the jab.

Mrs Thomas and her husband Ian, both 48, have decided to take on vaccine maker Merck Sharp and Dohme Limited.

A spokesman for the firm said all evidence indicates the MMR jab is not linked to the development of autism.

The couple hopes to prove in the High Court the combined MMR vaccine given more than 10 years ago damaged the boys who both now suffer from neurological damage and bowel disease.

Michael was given the MMR jab when he was 14-months-old.

'Piercing scream'

Mrs Thomas said she noticed he was experiencing difficulty shortly after he had the vaccine.

"It was obvious that something was wrong very quickly, he fell asleep and woke about an hour later and let out a horrendous piercing scream and he was in a lot of pain," she said.

The infant's condition gradually deteriorated from that moment onwards and he was eventually diagnosed with autism," she said.

According to Mrs Thomas a similar pattern of illness occurred in her other son Terry shortly after he received the MMR jab a year later.

"There's absolutely no doubt in my mind that the MMR vaccine was responsible for what's happened to my boys," said Mrs Thomas.