The aggregate value of all cryptocurrencies hit a record high of over $200 billion on Wednesday, according to industry website Coinmarketcap, putting their reported market value at more than that of U.S. banking giant Citigroup.

The new record came as the biggest and best-known cryptocurrency, bitcoin, hit a record high of $7,500 on the Luxembourg-based Bitstamp exchange, after a more than tenfold increase in value over the past 12 months.

That took its own "market cap" - its price multiplied by the number of coins that have been released into circulation - to a record high of more than $120 billion.

The second-biggest cryptocurrency, ether - sometimes known as "Ethereum" after the project behind it - has a market cap of just below $30 billion, with another 1000 or so rival digital currencies making up the rest of the $200 billion.

If the cryptocurrency market were a company, its valuation would put it in the top 25 firms on the S&P 500 stock index.

The latest surge in bitcoin was driven by news this week that CME Group, the world’s largest derivative exchange operator, would launch bitcoin futures in the fourth quarter of the year, as well as speculation that Amazon could be set to accept the digital currency.

Many are concerned that the market represents a bubble, with the latest warning coming from the head of Credit Suisse on Thursday.