Hackers are holding travel money provider Travelex to ransom after infiltrating its systems on New Year's Eve.
The cyber attack forced the firm to turn off all computer systems across 30 countries, after criminals broke into its sites with the infamous Sodinokibi ransomware.
Hackers have now reportedly demanded $3million (£2.3million) in exchange for either restoration of IT systems or the preservation of customer data.
They are reportedly threatening to release 5GB of customers' personal information - including social security numbers, dates of birth and payment card details - into the public domain unless the company pays up.
It is understood that a deadline for payment has been set by the cyber criminals.
The Metropolitan Police is leading the investigation into the attack.
In a statement, the force said: "On Thursday, 2 January, the Met's Cyber Crime Team were contacted with regards to a reported ransomware attack involving a foreign currency exchange. Enquiries into the circumstances are ongoing."
Travelex says it is working with police and has deployed teams of IT specialists and external cyber security experts who have been working continuously.
Travelex websites across Europe, Asia and the US have been offline since 31 December with a message to visitors that it is down due to "planned maintenance".
It means the large network of other firms that use its services cannot currently sell currency online.
Virgin Money's site showed an error message, which said: "Our online, foreign currency purchasing service is temporarily unavailable due to planned maintenance. The system will be back online shortly."
Sainsbury's Bank also said its online travel money services were unavailable, although it said customers could still buy travel money in its stores. In a statement to the BBC, the bank said: "We're in close contact with Travelex so that we can resume our online service as soon as possible."
A spokesperson for First Direct, which is owned by HSBC, said: "Unfortunately, our online travel money service is currently unavailable due to a service issue with third party service provider, Travelex."
In a statement on Thursday, Travelex boss Tony D'Souza said: "We regret having to suspend some of our services in order to contain the virus and protect data."
"We apologise to all our customers for any inconvenience caused as a result."
