Videos of unhygienic pranks at sushi belt restaurants in Japan have forced chains to instate new dining rules for customers and threaten legal action.

The phrase "sushi terrorism" emerged on Japanese social media this week after a video went viral showing a teenage boy covertly licking soy bottles on a sushi conveyor belt and putting them back.

The video, taken at a Sushiro diner in Gifu city, appears to have inspired copycats.

This has sparked a huge outcry online.

On Twitter, Japanese users described the sabotage as "sickening" and expressed fears about eating at sushi train restaurants. Many also criticised offenders - who appear to be mainly children.

"What did you do to my favourite sushi?" one user wrote. Another cried: "Where did our morals go?"

The videos have caused an uproar in Japan, known for its high standards of cleanliness.

The Gifu video on 29 January was followed by other videos - some with older timestamps. These have gained more than 40 million views. One showed customers spoiling passing sushi plates by heaping wasabi on top of them. Another showed a diner licking the spoon in a communal green tea powder container.

The Gifu incident prompted a 5% slump in the Sushiro parent company's stock price earlier this week, although it later recovered.

In a statement, Sushiro noted the teenager in the first viral video had apologised but the chain had launched a formal police complaint.

"As a company, we will continue to respond firmly with both criminal and civil cases," Sushiro said.

It said it had replaced all soy sauce bottles at the targeted Gifu store, and was instating new dining rules for customers as a result of the prank.

Customers in the region will now have to take utensils and condiments to their tables from a service point. Diners will also be able to request disinfected tableware.

Two other affected restaurant chains, Hama-sushi and Kura Sushi, also plan to take legal action, Japanese media reported.

Kura said it had plans to install cameras above conveyor belts to monitor customers, Jiji press agency reported.

Conveyor belt sushi restaurants have been a staple in Japan for decades and there were expressions of support for the restaurant chains online this week. Some tweeted messages with the hashtag #saveSushiro.

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