LONDON (AP) — Phone and Internet service provider TalkTalk said Friday that it has received a ransom demand after a “significant and sustained” cyberattack that may have compromised customers’ credit card and banking details.

Chief Executive Dido Harding said the company did not know how much data was stolen in the attack on TalkTalk’s website, but it was operating on a worst-case scenario in which “all of our customers’ personal financial information has been accessed.”

She said she had “received a contact from someone purporting … to be the hacker, looking for money.”

Harding said she did not know whether information on the company’s 4 million customers had been encrypted.

London’s Metropolitan Police said its cybercrime unit was investigating “an allegation of data theft from a telecommunications website” reported on Wednesday.

Talk Talk has acknowledged that it realized it was under attack on Wednesday. It has been criticized for not telling customers for more than a day.

It is the third known cyberattack in a year on TalkTalk, which provides mobile phone, broadband Internet and pay television services.

Shares in the company plunged more than 7 percent Friday morning to 248.4 pence ($3.83) on the London Stock Exchange.