A woman is urgently trying to trace the son she gave up for adoption 36 years ago to warn him that he may have a fatal hereditary disease.

Wendy Watson had her first son adopted in 1966 when she was aged 20, but went on to marry the child's father, with whom she had three further children.

Her husband is now suffering from the degenerative and incurable disorder, Huntington's disease, and one of their three younger children has also been diagnosed with the illness.

Mrs Watson, who now lives in Dorset, wants to warn her eldest son that he has a 50/50 chance of contracting Huntington's disease.

Symptoms of the disease include depression, mood swings, involuntary movement of the limbs, slurred speech and impaired judgement.

But Mrs Watson, whose surname has been changed to protect the identity of her three younger children, cannot get in contact with her first son, who she called Charles, because of rules making it impossible for her to reach him.

She said: "I only want to warn my son of the danger he may be in, so that he can make an informed decision about having children and about his own future. There must be lots of children with a genetic inheritance they ought to know about."

Childlink, the adoption agency that placed Mrs Watson's son, has contacted his adoptive mother to tell her of his medical history, but she is understood to have said she will not pass on the information to avoid worrying him.

Earlier this week, Childlink's chief executive, Caroline Hesslegrave, said she could not comment on the details of this specific case.

But she did say: "We do what we can to pass on information that is of potential significance about genetic disorders and family history to adopted people, but there is a limit to how far we can go."

She said the agency had limited funds to trace people, but where birth families were determined it was possible for them to pay for a tracer to be employed who would try to find the adopted person to find out if they would agree to contact being made.

"We have birth relatives who contact us all the time. Sometimes they have given up children for adoption as long as 60 years ago or more," she said.

"We would wish if possible to let adopted people know that contact was wished for by birth parents."

Sue Watkin, of the Huntington's Disease Association, said: "It isn't fair to keep from someone information that might affect their lives and their decision making.

"In our experience people who are not told about the condition become very angry if they only find out once they start experiencing symptoms.

"If a person has Huntington's they would probably want to think about whether they wanted to have children and whether they wanted to do things like take out a pension."

Huntington's disease used to be known as Huntington's Chorea or HC. The disease is a hereditary disorder of the central nervous system which usually develops in adulthood and can cause a very wide range of symptoms. The disease affects both men and women.