But officials suspicious about a couple in their 50s having babies started to ask questions.

Unable to have children, the Kondos had gone to a fertility clinic in the United States that introduced them to an egg donor and a surrogate mother.

Last October, they became parents of twin boys.

But with no laws on surrogate births, Japan ruled that the boys were not the Kondos' children.

The couple, who want rights as parents and citizenship for their sons, are in the midst of a legal fight with the government.

'We want this whole idea of the parent-child relationship reviewed,' said Mr Kondo.

Born to a surrogate mother overseas, the Kondos' twins are not regarded as Japanese citizens by the government. -- AP

The case is testing a system that has no provision for births using modern fertility techniques.

Surrogate births are opposed by the Japan Society of Obstetrics and Gynaecology, which sets ethical standards restricting in-vitro insemination to married couples.

With few doctors willing to offer surrogate services, many couples unable to have children have gone to the US, South Korea and other countries that have well-established practices.

Most of them have escaped being challenged when registering the births of their children.

But the Kondos were snared by a 40-year-old rule that requires checks on the children of couples aged above 50.

The Kondos can legally adopt the twins and apply for a change of citizenship, but Mr Kondo hopes the government will give in and accept them as the parents.