The Greek decision to ban the short selling of stocks follows a depressing pattern of regulators losing their confidence in the market and doing the wrong thing just when they should be providing a good example.

Prices on the Athens Stock Exchange have fallen by about 25% in the last month, according to index provider MSCI, and it's not surprising the Greek authorities want to do something. There is a terrible prospect of individual Greek shareholders panicking and selling their shares while their prices temporarily in the doldrums, and banning short selling must feel like a sensible way of protecting investors. But it's the wrong thing to do.