The UK's biggest high street banks have been hit by a cyber attack on Travelex, with Royal Bank of Scotland, HSBC and Barclays among those left with no online travel money services.
More than a dozen of the major banking players, also including Lloyds Banking Group and Virgin Money, are reporting that their online foreign currency systems are down following the New Year's Eve ransomware attack on Travelex.
Many are offering customers services in branches, but orders cannot be processed online.
Travelex was forced to take all its global websites offline and is reportedly being held to ransom by the infamous ransomware gang called Sodinokibi, also known as REvil.
It is understood the criminals are demanding cash - speculated to be some six million US dollars (£4.6 million) - and is reportedly threatening to release 5GB of customers' personal data - including social security numbers, dates of birth and payment card information - into the public domain unless Travelex pays up.
Travelex is the world's largest retail currency dealer and provides travel money services for a host of partners, also including the likes of Sainsbury's Bank and Tesco Bank.
Travelex owner Finablr, which is based in the United Arab Emirates, said late on Tuesday it is not expecting a "material financial impact" from the online attack.
Travelex has opened an investigation and confirmed in the update that while there has been some data encryption, and the extent is not yet known, there is no evidence that structured personal customer data has been breached.
But some of its business partners have spoken to the PA news agency of their frustration at the lack of information from the company.
Travelex had also not yet formally reported a data breach to the Information Commissioner's Office (ICO).
Organisations must notify the ICO within 72 hours of becoming aware of a personal data breach, unless it does not pose a risk to people's rights and freedoms.
A company which fails to comply can face a fine of up to 4% of its global turnover, under General Data Protection Regulations.
Travelex has said it is working towards total recovery after successfully containing affected areas and has managed to restore a number of internal systems.
As part of a detailed forensic analysis, the firm added there is also no evidence that data has been transferred from the Travelex system, known as exfiltration.
In a statement on Tuesday night, Travelex chief executive Tony D'Souza apologised for the inconvenience to partners and customers.
He insisted the group was "working tirelessly to bring our systems back online".
A joint investigation between the National Crime Agency and the Metropolitan Police is ongoing.
London-headquartered Travelex has a presence in more than 70 countries and more than 1,200 branches and 1,000 ATMs worldwide.
It processes more than 5,000 currency transactions every hour.
The PA news agency has confirmed that foreign exchange services are currently unavailable at the following 13 banks and one pawnbroker.
A Barclays spokesperson said: "Unfortunately we are unable to process foreign currency orders due to an issue with our service provider, Travelex. We are sorry for the inconvenience and will be restoring the service as soon as we are able to do so."
The bank said: "Our travel money purchasing service is temporarily unavailable."
It said: "Unfortunately, our online travel money service is currently unavailable due to a service issue with third party service provider, Travelex. We are not currently able to take travel money orders online or via telephone banking. We apologise for any inconvenience."
Lloyds, the parent of Halifax, said that the service had been lost.
HBOS's parent Lloyds confirmed that the service was down.
The bank said on its website: "Unfortunately, our online travel money service is currently unavailable due to a service issue with third party service provider, Travelex. We are not currently able to take travel money orders online or via telephone banking. We apologise for any inconvenience."
Its website said: "UNDER MAINTENANCE"
The bank confirmed to PA that its service was down. A Lloyds Banking Group spokesperson said: "We are currently unable to take orders for foreign currency online and in branch, due to the ongoing issue with our third party supplier, Travelex. Customers can visit our branches to purchase currency which may already be in stock. We are sorry for any inconvenience this may cause."
The bank, which is part of the RBS group, said: "We are currently unable to accept any travel money orders either online, in branch or by telephone due to issues with our Travel Money supplier, Travelex. We apologise for any inconvenience caused."
The Royal Bank of Scotland
RBS said: "We are currently unable to accept any travel money orders either online, in branch or by telephone due to issues with our Travel Money supplier, Travelex. We apologise for any inconvenience caused."
The bank said on its website: "Our online service is temporarily unavailable. Our in store travel money bureaux are open and stock most major currencies. We recommend you contact our team to check the availability of the currency you need. We're sorry for the inconvenience."
The bank said on its website: "We're sorry that we can't offer online ordering for Travel Money at this time.
"We have over 360 in-store Tesco Travel Money bureaux that are open as normal and hold stock for our top currencies. We recommend that you contact them first to check the availability of the currency that you are interested in."
The bank said on its website: "Our travel money purchasing service is temporarily unavailable. We apologise for any inconvenience caused and if you have any questions please call on."
It said: "Our travel money purchasing service is temporarily unavailable."