Billionaire investor George Soros of Soros Fund Management at the annual meeting of the World Economic Forum in Davos, Switzerland, in 2013. REUTERS/Pascal Lauener Holland's markets regulator said it "inadvertently" posted details on its website of short positions held by hedge funds in the Netherlands, including those of billionaire George Soros, according to a report in the Financial Times.

Hedge funds take short positions when they expect the price of a particular asset to fall. In other words, they are betting against the stock.

The details, which were briefly posted on Tuesday, showed Soros to have bet against the Dutch bank ING, taking a short position of 0.3% in the lender in June 2016, the Financial Times reported. The short bet was one of hundreds listed on the site going back to 2012, according to the Financial Times.

The markets regulator, AFM, also revealed that the secretive Medallion Fund, run by Renaissance Technologies for its employees, shorted numerous small-cap Dutch stocks.

The Medallion Fund is one of the most successful quantitative trading funds ever, making an annualised return of over 70% in the 20 years from 1994 to 2014.

European Union rules in force since 2012 require hedge funds to disclose to the public short positions of over 0.5% in a particular company's stock. Positions of over 0.2% have to be made known only to the domestic markets regulator, updated for each additional 0.1% — it is these nonpublic trades that the AFM briefly revealed.

"On the afternoon of Tuesday 24 January, after the close of the market, the AFM inadvertently published a list on its website that included net short positions of less than 0.5% instead of publishing the daily list of net short positions of 0.5% and higher," the AFM said in a statement on its website.

"The AFM corrected this mistake and posted the correct list of net short positions of 0.5% and higher on the morning of Wednesday, 25 January. We regret this error."

A successful short trade often involves borrowing the stock of a company from an investor, selling it, and then buying it back at a later date for a lower price, pocketing the difference.

Soros, who made a $1 billion betting against the British pound in 1992, is known for his short trading. A day after the UK's June 23 referendum to leave the European Union, he took out a $108 million short position in Deutsche Bank.