More than 230,000 Japanese people listed as 100 years old cannot be located and many may have died decades ago, according to a government survey released today.

The justice ministry said the survey found that more than 77,000 people listed as still alive in local government records would have to be aged at least 120, and 884 would be 150 or older.

The figures have exposed antiquated methods of record-keeping and fuelled fears that some families are deliberately hiding the deaths of elderly relatives in order to claim their pensions.

The nationwide survey was launched in August after police discovered the mummified corpse of Sogen Kato, who at 111 was listed as Tokyo's oldest man, in his family home 32 years after his death.

Kato's granddaughter has been arrested on suspicion of abandoning his body and receiving millions of yen in pension payments after his unreported death.

Soon after came the discovery that a 113-year-old woman listed as Tokyo's oldest resident had not been seen by her family for more than 20 years. Welfare officials have yet to locate Fusa Furuya, who was last seen in about 1986.

Japan's failure to maintain accurate records of its oldest citizens is also being blamed on strict privacy laws and weakening family and community ties. "It appears that these people were isolated, given that it is unclear whether they are dead or alive despite the family registration system," the justice minister, Keiko Chiba, told reporters.

The survey uncovered 234,354 centenarians who are listed as still alive but whose addresses could not be confirmed. Ministry officials suspected some deaths went unreported in the confusion that followed the end of the second world war, while other people may have lost touch with relatives or moved overseas without informing the authorities.

The discovery has proved a major embarrassment in a country that supposedly reveres its oldest citizens.

Every September people who have just turned, or are about to turn, 100, receive a congratulatory letter and a trophy from the prime minister.

The debacle is partly a symptom of the bureaucratic struggle to maintain accurate records in one of the world's fastest-ageing societies: more than one in five Japanese are aged 65 or over.

According to the health ministry, the country has 40,399 centenarians with confirmed addresses, more than triple the number a decade ago. Japanese women can now expect to live an average of 86.4 years – the longest life expectancy in the world – while the average among men is 79.6 years.

The government said the findings would have a minimal impact on longevity figures, which are based on census data collated during home visits. In addition, men over 98 and women over 103 are not factored into life expectancy calculations.