The owner of Carphone Warehouse and Dixons has warned that more expensive mobile phone handsets and lower EU roaming charges will hurt its profits.
Shares fell more than 30% after it said profits this year would be £360m to £440m, down from £501m last year.
Dixons Carphone said customers were not upgrading their phones as frequently because handsets had risen in price.
It also predicted it would take a hit from the EU scrapping roaming charges for people using mobile phones abroad.
Chief executive Seb James said in an unscheduled statement that the market for contract mobile phones had become “more challenging” in recent months.
The fall in the pound had made handsets more expensive and innovation of phones has been “incremental”, Mr James said.
As a result, phone users are typically holding onto their handsets for four to five months longer, Dixons Carphone’s executives said.
It is hoping that the next iPhone 8 – due to be launched in the autumn – will prove a hit after its sales of the iPhone 7 were disappointing.
Mr James said: “We’re anticipating that iPhone 8 launch will be much better, maybe not as good as the iPhone 6, more like the same numbers as the iPhone 6S.
“We think the iPhone 8 is a better phone and we will see our customer base returning.”
It expects the new Samsung Note 8 – unveiled on Wednesday at a price of £869 in the UK – to have less of an impact.
“We don’t think the Note 8 is causing an enormous amount of disruption in the market, it’s a nice phone but a bit small [in terms of impact],” Mr James said.
Dixons Carphone, which takes a slice of the value of customers’ phone contracts, also said it would take a hit of up to £40m from the changes to EU roaming legislation in force since June.
Mr James said it was “hard to quantify at this stage” how big the impact would be on the business.
The hit from lower roaming charges compares with one-off benefits of £71m last year, and the retailer said its core retail business would remain steady this year.
It expects growth in its UK and Ireland, Nordic and Greek electrical businesses to offset the falls in mobile phone sales.
Dixons Carphone shares also went ex-dividend on Thursday.
That often leads to a fall in the share price as it will be at least six months before the next dividend is paid out.