A mother who believes her sons were harmed by the controversial MMR vaccine has accused a drug company of trying to bully her into dropping a legal action.

Isabella Thomas was sent a letter from solicitors acting for Merck warning her she faces substantial legal costs if she continues her battle.

She is one of hundreds of parents suing drug companies for compensation because of medical problems they claim their children developed after vaccinations.

Two of her sons, Michael, 12, and Terry, ten, are severely autistic and have a string of other chronic viral conditions which Mrs Thomas believes are linked to the MMR jab.

She and other parents received the letter from solicitors Lovells saying they had until the end of the month to decide whether to waive all rights to claim compensation or face hefty legal bills.

Mrs Thomas, from Newhaven, who is a spokeswoman for campaign group Jabs, said: "They can't intimidate us. They can take everything from us. They can take our home but we will keep on fighting.

"It may sound like strong wording but I would fight to the death for my sons. All the parents would. We've been through such a lot already. We're not going to give up now."

The letter names the boys as being responsible for potential legal costs if they lose the case.

It reads: "From the moment Michael and Terry cease to be legally aided they and you, as a litigation friend, will be at risk of facing a liability for any further costs Merck continues to incur while defending the claim."

If parents choose to back down by sending back two blank discontinuance notices to Lovells, the company says it will not seek costs.

Mrs Thomas said: "When I read the letter I was so angry. One of the lines that made me laugh in disbelief said, 'By this means Michael, Terry, you and Merck can all achieve finality'. What kind of finality can my sons get?"

The boys have different types of autism as well as other health problems. Michael has also developed obsessional compulsive disorder, has memory problems, hallucinates, gets serious headaches and mental blackouts and has breathing difficulties, eye problems, chronic bowel disease and possible epilepsy.

Mrs Thomas said: "Very high levels of the measles vaccine have been found in his blood stream.

"Each morning I wake up I think, 'God, what's going to happen today? Is Terry going to get jaundice again? Is Michael going to have another fit? Will he try to open the car door when we're driving and jump out again to try to stop the pain in his head? Will he hallucinate again? Will he have such a severe tantrum he breaks the furniture?' It's 24 hours a day.

"It's heartbreaking watching your children in pain every day. It's terrible knowing there's therapy and help but no one will give it to them.

"I know doctors who've said, 'We'd like to treat your boys but we want to keep our jobs'. People are scared because they don't understand it and they don't know what to do.

"It'll be the taxpayer who has to fork out for medical help when these children are adults. It shouldn't be. It should be the drug companies."

Michael was 14 months and Terry 21 months when they had the MMR jab.

Mrs Thomas has two older sons, Ian, 26, and Nigel, 24, neither of whom had MMR and who are both healthy.

She said: "Michael had a severe reaction within a few hours. We knew he was very sick and we thought it might have been the vaccine but no one would listen to us.

"Michael would scream for hours. It was this high-pitched screech that would get higher and louder - nothing like normal crying.

"One day I phoned the clinic while he was screaming to ask if it was to do with MMR. The nurse said, 'Pull yourself together. Some babies do that'."

Mrs Thomas, 47, and her husband Ian, 46, are among about 1,000 families fighting to retain legal aid in order to sue drug companies.

Last year lawyers acting for the families lost an appeal against a decision by the Legal Services Commission (LSC) to withdraw legal aid.

The LSC said medical research had not provided a conclusive link between the vaccine against measles, mumps and rubella (MMR) and autism.

However, Mrs Thomas believes scientists are on the brink of finding the necessary proof.

Lovells said 500 letters had been sent so far and 50 parents had accepted the deal. A spokesman said: "The letter was sent with the purpose of making an offer and was not meant to be intimidating, bullying or threatening. Some parents have been grateful to accept the offer."