Seven diners in northern Japan fell ill and three were still in hospital today after eating blowfish testicles prepared in a restaurant not authorised to serve the poisonous delicacy.

The owner of the restaurant in Tsuruoka city, who is also the chef, had no licence to serve blowfish and was being questioned on suspicion of professional negligence, police official Yoshihito Iwase said.

Iwase said the seven men ordered sashimi and grilled blowfish testicles at the restaurant last night.

Shortly after, they developed limb paralysis and breathing trouble and started to lose consciousness – typical signs of blowfish poisoning – and were rushed to a hospital for treatment.

A 68-year-old diner was in a critical condition with respiratory failure and two others, aged 55 and 69, were in serious condition, Iwase said.

"It's scary. If you go to a decent-looking restaurant that serves fugu, you would assume a cook has a proper fugu license," Iwase said, using the Japanese term for blowfish.

Blowfish poison, called tetrodotoxin, is nearly 100 times more poisonous than potassium cyanide, according to the Ishikawa health service association. Eating it can cause death within an hour and a half.

Three people died and 44 others became ill from blowfish poisoning in Japan in 2007 – most of them after catching the fish and cooking it at home, according to the health ministry.