TalkTalk says the final bill for dealing with last month's hacking of its customers' private information could reach £35 million.
Revealing its interim financial results for the six months to 30 September, the company said it had set aside £30–35 million to deal with the crisis.
The company said this week that a total of 156,000 customers' details were accessed, including 15,000 bank account numbers and sort codes.
TalkTalk said "significantly fewer customers [were] affected than previously feared".
As a sweetener for angry customers who were either directly affected or worried by the affair, all customers are being offered a free upgrade to a better TalkTalk package.
TalkTalk CEO Dido Harding said in a video message to investors that it was impossible to gauge the long-term impact the attack would have on the business.
"While the number of customers directly affected is materially smaller than we at first feared," she said, "I would in no way wish to diminish the impact of the unavoidable uncertainty of the attack on all of our customers.
"We're doing everything in our power to ensure that going forward our customer data is secure.
"In terms of the longer-term impact? It's far too early to tell the impact on the business in the long run. But I am personally humbled and encouraged by feedback from the majority of our customers who said TalkTalk is doing the right thing."
Four people, three of them teenagers, have been arrested in connection with the attack.
The latest arrest last week, of a 16-year-old boy from Norwich who was bailed until March 2016, came after:
— A 15-year-old boy from County Antrim, Northern Ireland, was arrested and bailed until an unspecified date in November.
— A 16-year-old from Feltham, southwest London, was arrested and released on bail.
— A 20-year-old man from Staffordshire was arrested and bailed until early March.
All four were arrested on suspicion of crimes under the Computer Misuse Act.
Despite the "one-off financial impact", the company says it's on course to meet full-year market expectations set before the hacking.
The company added 176,000 customers during the period and said revenue was up 4.7% compared to the same period last year.
In its last full financial year, to 31 March 2014, the company made pretax profits of £213 million, on revenue of £1.72 billion.
Patrick Smith is a senior reporter for BuzzFeed News and is based in London.
Contact Patrick Smith at email@example.com.
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