Bill Gates, the billionaire founder of Microsoft, today emerged as a surprise investor in troubled sportswear retailer JJB Sports.

JJB's share price jumped 17% when it was revealed that Gates bought 800,000 shares in his own name and through his charitable foundation, the Bill & Melinda Gates Foundation Trust, two days ago.

This took his holding in JJB to 3.14%, above the 3% level when share purchases have to be disclosed. JJB announced the investment in a statement to the London stock exchange today.

Shares in JJB gained 4.25p to 29.25p.

The purchase is good news for JJB, which wants to shore up its battered finances with a £50m rights issue. There was speculation that Gates would buy more shares and participate in the fundraising.

"It obviously doesn't look bad to have him on the shareholder register," said one trader. "There's rumours going around that he wants to increase his stake and he must see something in the company. I think it'll be over 30p by the end of the day."

It is not the first time Gates has invested in a small British retailer. In May 2008, he took a 3% stake in Carpetright.

JJB has been engulfed in a row between executive chairman David Jones, and Mike Ashley, who owns the rival Sports Direct chain, over a personal loan Jones received from Ashley two years ago.