A criminal investigation has been launched into a mystery UK entity believed to be involved in the massive money laundering scandal centered around Danske Bank.

The UK's National Crime Agency said it was "aware of the use of UK registered companies in this case and has related ongoing operational activity."

Danske Bank's money laundering scandal relates to non-resident transactions at its Estonian branch, and could have involved as many as $235 billion worth of transactions.

CEO Thomas Borgen resigned from the helm of the bank earlier this week as a result of the scandal.

The British entity tasked with dealing with serious and organised crimes has launched a probe into a mystery UK firm that is potentially linked to the massive money laundering scandal which this week forced the resignation of Danske Bank's CEO.

It was reported overnight that the National Crime Agency has launched a criminal investigation into a UK entity which is believed to have links with the Estonian branch of Denmark's biggest bank, which is at the centre of the scandal that could potential have involved $235 billion of cash flows.

Investigations centre on a "UK-registered limited liability partnership, or LLP," according to a report from the Financial Times, which cites people familiar with the investigation. The entity in question has not been named.

"The NCA is aware of the use of UK registered companies in this case and has related ongoing operational activity," the National Crime Agency said in a statement provided to the FT.

"The threat posed by the use of UK company structures as a route for money laundering is widely recognised and the NCA is working with partners across government to restrict the ability of criminals to use them in this way," it added, declining to comment further.

The investigation into suspicious transactions at Danske Bank's Estonian branch centers on so-called nonresident transactions — effectively transactions done by people not based in Estonia but using the bank's facilities there.

It said it had identified about 10,000 customers who fit the profile of nonresidents, with 6,200 of those fitting what the bank called "the most risk indicators." Of these customers, Danske Bank said, the "vast majority have been found to be suspicious."

It did, however, emphasize that just because a customer had "been found to have suspicious characteristics does not mean that there is a basis for considering all payments in which the customer in question was involved to be suspicious."

As well as the initial 10,000 customers, a further 5,000 customers with nonresident characteristics have also been identified.

In total, Danske Bank says, these 15,000 customers undertook about 9.5 million payments, with the total value of the money flowing about 200 billion euros, or $234 billion.

Analysts believe that the scandal could end up costing Danske Bank as much as $8 billion in fines, if it is found to have committed any wrongdoing