A 72-year-old man is due to become the father of his own "grandchild" by acting as a sperm donor for his daughter-in-law. The case is thought to be the first of its kind in the UK.

The government's fertility watchdog said there was a "handful" of men over 65 on the sperm register, but some fertility experts are worried that the couple are being subjected to undue risks because the sperm is likely to have many genetic mutations accumulated during the donor's life.

"By the time you get to 70 the risks are significantly stacked that there will be errors in there," said Allan Pacey at the University of Sheffield, who is secretary of the British Fertility Society. "That could lead to failure of the procedure. It might mean a greater chance of miscarriage."

The child would also be at greater risk of genetic diseases such as Down's syndrome and autism, he added. "My professional advice would be donors should be under the age of 40 wherever possible, but there are always going to be exceptional circumstances."

After failing to have a child naturally the couple sought fertility treatment and discovered sperm problems. Doctors at their Harley Street clinic offered to provide sperm from an anonymous donor, but the couple wanted to use a member of their own family.

"We have made the decision on the basis that the couple have special requirements in that donor sperm is not acceptable to them," said Kamal Ahuja of the London Women's Clinic. "That applies to many, many groups of people - some religions [such as Islam] don't condone the use of donor sperm. In this particular case there was a mixture of reasons." The decision was referred to the clinic's ethics committee, which considered whether going ahead would be in the long term-interest of the child. The wife's mother-in-law has agreed to the procedure.

According to guidance from the Human Fertilisation and Embryology Authority, sperm donors cannot be older than 45 "unless there are exceptional reasons". Of the nearly 6,000 men on the donor register only around 300 are over this age. Guidelines from the British Andrology Society are even more conservative, citing 40 as the recommended upper age limit.

"The age limit doesn't apply in quite the same way [in this case] because they have said we want that person for family reasons. The clinic would have to make a clinical decision based on is this a suitable treatment," said a spokesperson for the HFEA.

It is not uncommon for families to use a sperm donor from within their family - often a brother - so that the child will have genetic ties with his or her "father", but this is thought to be the first case of a grandfather acting as a sperm donor. The family have not yet decided whether they will tell the child who his or her genetic father is, although the clinic is encouraging them to be open about that. "That's their personal decision," said Dr Ahuja.