Bitcoin bubble warnings issued as futures trading opens in Chicago

First contract on a regulated exchange is seen as step towards legitimacy for the cryptocurrency but volume traded is tiny

Bitcoin has taken a step toward legitimacy with the launch of a product on a Chicago exchange that allows investors to take bets on its price in the future.

The new contract on the Chicago Board Options Exchange came as the digital currency jumped another 10%, sparking fresh warnings of a speculative bubble after last week's 40% climb in price.