Dixons Carphone headline pre-tax profits have slumped 22 per cent to £298 million.
The firm also said revenue was down 1% to £10.43 billion in the 12 months to April 27.
On a statutory basis, the group reported a loss before tax of £259m, after £557m in non-headline charges.
Dixons Carphone said headline profits would slide again in the 2020 financial year to £210m.
Chief executive Alex Baldock said: "In UK mobile, the market is changing in the way we described in December, but doing so faster.
Today we're sharing our preliminary results for the financial year 2018/19. Visit https://t.co/5u9rxWxzkO for more information. pic.twitter.com/US7VuSvVzo
— Dixons Carphone plc (@DixonsCarphone) June 20, 2019
"So, we're moving faster to respond: we've renegotiated all our legacy network contracts, we're developing our new customer offer and are accelerating the integration of mobile and electricals into one business.
"This means taking more pain in the coming year, when mobile will make a significant loss."
Shares in the company plummeted by more than a quarter in early trading on Thursday, dropping at one point to just 90p.
Homeware retailer Dunelm has upped its profit outlook for the second time in as many months after good weather helped boost sales.
The group said it had seen "very good" like-for-like sales growth since its latest update on April 10, particularly in May and June.
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It cited weaker comparative trading from a year earlier and "unseasonably favourable weather conditions this year".
The group now expects full-year pre-tax profits of between £124 million and £126 million.
Fashion retailer N Brown has posted further sales declines, but insisted its turnaround strategy is bearing fruit as online trade lifted.
The group said total revenue dropped 3.8% in the 13 weeks to June 1, dragged lower by a 5.4% fall in product sales from the managed decline of its traditional, mail order business.
Its shift away from catalogue business saw online sales rise 3% in its first quarter, with 83% of total revenue now digital.
Steve Johnson, chief executive of N Brown, said: "We're pleased to report a solid trading performance in the first quarter.
"In line with our strategy, we delivered digital revenue growth across JD Williams, Simply Be, Ambrose Wilson and Jacamo as we continue to improve our customer offer whilst managing the decline of our legacy offline business.
"The retail market remains challenging, but we have a clear strategy to deliver profitable digital growth and our full-year expectations are unchanged."
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