LONDON — French, Italian and Spanish stock market regulators decided Thursday to extend the temporary bans on short-selling introduced this month in a bid to stem market volatility.

Spain and Italy extended their bans through Sept. 30, regulators in both countries said in a statement. The French market regulator said its ban could last as long as Nov. 11.

The three countries, along with Belgium, imposed bans on short-selling, or bets on falling prices, of some financial stocks this month in an effort to stabilize markets after the shares of some European banks, like Société Générale, hit their lowest levels since the credit crisis of 2008. The restrictions cover the short-selling of shares and equity derivatives in some financial firms. Belgium’s financial markets regulator said Thursday that it would examine lifting its ban as soon as market conditions allow.

Short-sellers sell borrowed shares with plans to buy them back later at a lower price, a practice politicians and some investors blame for roiling markets. The initial restrictions introduced by France, Spain and Italy were temporary, lasting 15 days. Belgium’s is indefinite. Traders had worried that Germany might follow in the footsteps of France, Spain and Italy and ban short-selling. But on Thursday, Dominika Kula, a spokesman for BaFin, the German markets regulator, said the agency had “all the regulation in place” regarding the short-selling of equities.

The German Finance Ministry, which oversees BaFin, also denied talk that it planned to extend the country’s ban on naked short-sales to all transactions. In such transactions, a trader bets on a security’s fall without actually borrowing shares as in a normal short sale. That practice has been criticized for encouraging speculation.

German stocks and index futures tumbled Thursday as traders reacted to rumors that the country would widen its market controls, and also on a deteriorating outlook for the German economy.

By day’s end, the DAX index had pulled back 1.7 percent, after falling 4 percent earlier in the session.

Some market followers questioned the effectiveness of the bans, saying they would have little impact on equities. “Short-selling equities is not a significant danger to financial stability, so these bans are irrelevant,” Richard Portes, a professor of economics at London Business School, wrote by e-mail.

The bigger problem, said Mr. Portes, were derivatives that targeted financial institutions and government bonds. “The serious problem is speculation against financial institutions and sovereigns using naked credit default swaps,” he said, referring to derivatives that enable investors to insure themselves against default. “They should be banned.”

Euro Stoxx 50, a barometer of European blue-chip stocks, has gained less than 0.1 percent since Aug. 11, the day the bans were announced. It was down almost 1 percent on Thursday.

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