Revenues of £1.43bn for the six months to 25 October and underlying earnings of £218.5m were below  analysts' forecasts, making the stock the worst performer in the FTSE 100.
The index closed down by 38.63 points at 6,088.05.
Shares in financial services group Old Mutual, which has interests in South Africa, sank more than 10%.
South African's President, Jacob Zuma, sacked Finance Minister Nhalnhla Nene late on Wednesday, and South Africa's currency fell close to a record low against the dollar in reaction.
Shares in mining giant Glencore jumped 7% after it announced plans to speed up the rate at which it cuts its debts.
The company now aims to cut its debts to $18bn-$19bn (£11.8bn-£12.5bn) by the end of 2016, down from a previous target of "low $20s billion".
British Gas owner Centrica rose 2.6% after the energy firm said it was on track to meet expectations for full-year earnings.
Investors welcomed full-year results from holiday group Tui, sending their shares up 4.6%. The company - which owns the Thomson and First Choice - reported a 23% jump in full-year underlying earnings to €1.07bn (£775m) and gave an upbeat outlook for the coming year.
On the currency markets, the pound slipped 0.06% against the dollar to $1.5172, but was up by nearly one cent, or 0.68%, against the euro at €1.3861.
(Close): Sports Direct shares dived 11% after the retailer's half-year results fell short of expectations.