Shares in Dixons Carphone have closed 20.7% down after it warned of a sharp fall in profits this year.
The mobile phone and electrical goods retailer also said it would close 92 of its more than 700 Carphone Warehouse stores this year.
It expects pre-tax profits for 2017-18 to be £382m, but it predicts profits will fall to £300m in 2018-19.
Chief executive Alex Baldock said "nobody is happy with our performance" but the problems were all "fixable".
The company blamed "challenges" in the market for mobile phones and mobile services, including a declining market for long-term mobile contracts and people not renewing their handsets as frequently.
It had also seen weaker demand for computers.
However, total sales were 3% higher in the year to 16 April, while like-for-like sales were up 4%.
In the UK, sales grew 2% for the year as a whole, and by 1% in the fourth quarter.
The international division did better, with like-for-like sales in Nordic countries up 9% in the year and Greece up 11%.
'Plenty to fix'
Mr Baldock, who took over as group chief executive earlier this year, said the international business was in "good shape" so "we're focusing early action on the UK".
"We won't tolerate our current performance in mobile, or as a group. We know we can do a lot better," he added.
"Eight weeks in the business have cemented my optimism about Dixons Carphone's long-term prospects. I've found exceptional strengths, and though there's plenty to fix, it's all fixable."
The company expects a further decline in the market for long-term mobile contracts in the current financial year, but forecasts an increase in its share of the market for SIM-free handsets.
It was also "budgeting for a contraction" in the UK electrical goods market.
Neil Wilson, chief market analyst for Markets.com, described the profits warning as "grim" but agreed the problems were "all entirely fixable".
"Dixons looks a bit flabby, and the market is just as soft, but there should be some easy wins in terms of making it leaner, especially around store closures," he added.
Dixons said no jobs would be lost as a result of the store closures.