Parents who learned only by accident that they could claim damages for doctors' errors at their son's birth nearly 38 years ago are expected to win at least £3m compensation for him in a case which goes to the high court today.

The family of Richard Coman took legal action in 1996 after they read about a similar case.

Two years ago, Norwich health authority accepted 75% responsibility for brain damage caused by delay in delivering the baby. Lawyers for the Coman family will ask the high court to award £3.5m, while the health authority is offering £500,000 less.

Personal injury claims on behalf of children must normally be made by the age of 21. If the victim is mentally incapacitated there is no time limit.

Joan Coman, of Norwich, was admitted to West Norwich hospital in February 1963 when her son's birth was overdue. Doctors twice tried to induce the birth, but without success. Eventually she was transferred to the Norfolk and Norwich hospital, where the baby was delivered by caesarean section.

By then he was 22 days overdue, and an MRI brain scan showed that he had suffered a constriction to his breathing for several hours. He is severely disabled with a mental age of three and requires round-the-clock care.

Mrs Coman and her husband, Sidney, both in their 70s, have already received interim payments of £550,000. She said: "We brought this action because we wanted to ensure a secure and stable future for Richard and his extensive care needs."

Simon John, their solicitor, said: "One of Richard's problems is that he has no road sense, in fact no awareness of danger whatever. So it's been crucial to find a house where his parents can know he will be safe."