In a rare departure from the official line that the meat is safe and nutritious, two assembly members from Taiji in Wakayama prefecture broke ranks to say that tests on samples of short-finned pilot whales - a type of large dolphin, despite its name - had found mercury levels 10 to 16 times higher than those advised by the health ministry.

"In kindergartens, elementary schools and middle schools, children are served the meat two or three times a month, but their parents believe that it comes from whales caught in the Antarctic. They seem to be unaware that their children are eating these pilot whales," said Hisato Ryono, who described the meat as "toxic waste".

In some cases the mercury levels were higher than those recorded in seafood affected in the infamous Minamata mercury poisoning case, which has been responsible for the deaths of 1,700 people since the 1950s. Mercury poisoning can damage the nervous system and internal organs and is known to be dangerous to the foetus.

One of the Taiji samples, bought from two local supermarkets, contained almost 16 times more mercury than the ministry's accepted level of 0.4 parts per million, the councillors said. The same sample also contained 12 times more methyl mercury than is deemed acceptable.

Meat from the dolphins is currently only served in Taiji, about 280 miles west of Tokyo, but authorities plan to go ahead with the construction of a ¥330m (£1.4m) dolphin processing plant that will ship meat to other parts of the country as part of efforts to boost domestic consumption, particularly among children.

Mr Ryono and his colleague, Junichiro Yamashita, decided to go public with their findings after Taiji authorities ignored their requests to have the meat inspected before it was served in school lunches.

Their claims, however, have been ignored by most of the country's media. "The media have concerns because such information can have an impact on the fishing industry in Taiji," Mr Yamashita said. "But it is a problem because local residents, including the parents of schoolchildren, remain unaware [of the dangers]."

They warned that other dolphin and whale meat is unlikely to be safe for human consumption. "I believe that pollution in the sea around Japan is the cause of this," Mr Ryono said. "The fish are contaminated with mercury and then eaten by whales as part of the food cycle. The levels rise because whales accumulate mercury."

The claims come as Taiji prepares for its annual dolphin cull. The town slaughters about 2,300 of the 20,000 dolphins killed in Japanese waters every year over the next six months. In a practice condemned as barbaric by animal rights campaigners, fishermen drive pods of dolphins towards the shore and spear or hack then to death.

Although the councillors say they do not oppose traditional whale and dolphin hunting, they have been shunned by fellow assembly members since going public with their findings.

"The health ministry may not be taking this problem seriously but once the information spreads, the country will have to face the issue," Mr Yamashita said.