Shares of British Airways' parent company IAG fell around 4% as markets opened on Friday morning, hours after the airline said the credit card information of at least 380,000 customers had been "compromised" in a data theft.
"The breach has been resolved and our website is working normally".
The airline is advising those customers to contact their bank or card provider and follow their advice.
"British Airways customers will be concerned to hear about this data breach", said consumer group's Alex Neill.
He added that no passport data had been obtained in what he called a "very sophisticated, malicious criminal attack", but that British Airways is "100 per cent committed" to compensating customers.
The National Crime Agency said it was aware of the data breach affecting British Airways and was consulting with partners, including the National Cyber Security Centre, to assess the best course of action.
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The Royal Bank of Canada said it has not yet seen any impact on its credit card customers. There were other methods, very sophisticated attempts by villains to acquire these elements. "We will be contacting customers and will manage any claims on an individual basis".
Speaking to Sky News, he said the online vulnerability had now been fixed after it was discovered on Wednesday, and indicated compensation could be a possibility for anyone who had lost money.
He said hackers stole enough information to use bank cards to make purchases.
Customers' banking information was compromised, but no travel information. It is now vital that the company moves quickly to ensure those affected get clear information about what has happened and what steps they should take to protect themselves.
The incident comes after an IT meltdown caused huge disruption for BA passengers at the start of the May half-term holiday.