Britain's largest online trading platform, Hargreaves Lansdown, is to allow its customers to invest in bitcoin, the alternative digital currency.

The broker, which administers £70bn of investors' money, will give its 876,000 customers access to a fund that tracks the price of bitcoin, the best-known of a new breed of alternative currencies.

The fund, offered by a Swedish company, XBT Provider, is structured as an "exchange-traded note" (ETN). This is a fund listed on the stock exchange, meaning it can be bought and sold like a share.

XBT buys complicated "synthetic" certificates, listed in US dollars, that aim to track the price of bitcoin. The ETN itself is denominated in Swedish krona. British investors are therefore exposed to two levels of currency risk.

The Swedish firm warned: "The value of and any amount payable under the certificates will be strongly affected by the performance of bitcoin and the US dollar/krona exchange rate. As such an investment in the certificates is likely to be highly volatile and thus risky."