The group's shares plunged from 305p to 235p, cutting its value from pounds 120.5m to pounds 93m. Until yesterday, its shares had performed strongly, jumping from a low of 202p last October to a peak of 313p.
QS blamed the shortfall partly on rivals moving into its market. A number of retailers, including BhS and Littlewoods, are starting to re-brand some of their stores to focus on the discount market.
It has also been hit by sterling's devaluation, which has increased the cost of supplies sourced in dollars, and poor weather in April, May and June. There has been some improvement in sales so far this month, but QS warned: 'It is too early to comment on the outcome for the year as a whole.'
The group made pounds 3.9m in the six months to last July, rising to pounds 8.5m for the year as a whole.
Panmure Gordon, its house broker, has downgraded current-year forecasts from pounds 9.2m to pounds 5.5m, although it expects a recovery to pounds 8.5m the following year.
QS said it expected to keep its interim dividend at 1.56p and said it was 'strongly positioned to take advantage of any upturn in the retail economy'.