Shares in the Japanese operator of 7-Eleven fell Wednesday as reports suggested growing opposition within the firm to a takeover bid by Canadian rival Couche-Tard to create an international convenience store empire.

The purchase of Seven & i Holdings would be the biggest ever foreign takeover of a Japanese firm, combining 7-Eleven, Couche Tard, Circle K and other brands across Asia, North America and Europe.

The world's biggest convenience store chain, 7-Eleven operates more than 85,000 outlets globally, while in Japan it is a beloved institution, selling everything from ready meals to umbrellas.

Alimentation Couche-Tard (ACT) operates more than 16,700 in 31 countries and territories. 

Both firms confirmed the unsolicited bid earlier this month but gave no details on its size. 

Seven & i, with a market value of around 5.5 trillion yen ($38 billion), has formed a committee of outside directors to review the approach.

But the company has asked the Japanese government to designate parts of the company as "core", which would make a takeover more difficult, Bloomberg News and the Financial Times reported.

Brands with the "core" rating in Japan range from manufacturers in the nuclear, space, rare earths and chip industries to cybersecurity operators and key infrastructure service providers. 

Shares in Seven & i, which soared 19 percent on August 19 when the offer was announced, closed down 1.03 percent at 2,100 yen Wednesday.

"Any 100 percent takeover of a (core) company will require government approval," Travis Lundy, an analyst at Quiddity Advisors who publishes on SmartKarma, told AFP. 

But even so, the rating is "not a sure block. Many deals have been done".  

"There could certainly be investor pushback against management efforts to scuttle a deal, which was otherwise attractive to shareholders," he added. 

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