Denmark's top bank handled up to €28bn of dodgy Russian funds in just one year, in what has the makings of the biggest money-laundering scandal in European history.

The latest chapter in the unfolding story was written by Promontory Financial, a US-based consultancy hired by Danske Bank to look into the affair.

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"NRP [non-resident portfolio] transaction volume peaked in 2013 with the number of transactions approaching 80,000 that year, and the transaction volume approaching $30bn [€28bn]," the consultancy said in a report seen by the Financial Times, a British newspaper.

A non-resident client is a person who lives in one region or jurisdiction, but has financial interests in another one.

Not all the money was likely to have been illicit, but all of it came from Russian or ex-Soviet republic clients and all of it flowed through Danske Bank's tiny branches in Estonia, which ought to have rung alarm bells at head office in Copenhagen.

Revelations first came out in Danish newspaper Berlingske in September 2017 that Danske Bank had handled €2.4bn of shady Azerbaijani money between 2012 and 2014.

Further revelations in July this year showed it had also handled €7bn of dodgy Russian funds between 2012 and 2015.

These funds were linked to the family of Russian leader Vladimir Putin and to the Russian intelligence service, the FSB, a Danske Bank whistleblower said.

Some of them were also linked to the murder of Russian anti-corruption activist Sergei Magnitsky, the activist's former employer, British businessman Bill Browder, said, in what he called "blood money".

The bank received warnings from Danish and Estonian regulators and internal watchdogs in 2012 and 2013, but it shut down the Estonian operation, dubbed a "laundromat", only in 2015.

The €28bn figure dwarfs all previous scandals of the type in modern times in Europe.

British bank HSBC was found in 2012 to have handled €760mn of Mexican drug cartel funds.

Germany's Deutsche Bank was also outed last year for having handled up to €9bn of shady Russian transactions.

But the Danish scandal was "gigantic" in scale compared to these, Browder said on Monday (3 September).

It involved "a truly breathtaking amount", a source linked to Promontory Financial's investigation told the Financial Times.

With branches on many street corners in Danish cities and with its head office in a historic building on Kongens Nytorv, the central square in the Danish capital, the affair has already cost the bank dearly in reputational damage.

Its share value has dropped by 20 percent over the past year and some of its executives, including Lars Morch, head of international banking, have resigned.

The shares fell a further 6 percent on Tuesday after the Financial Times report.

The costs are set to increase amid an ongoing investigation by Danish authorities as well as a criminal complaint against 26 of its executives filed by Browder earlier this year.

The US, which fined HSBC $1.9bn (€1.6bn) back in 2012, might also fine the Danish lender, given that Danske Bank has a US dollar bond programme and handled US dollar transactions in Estonia.

"We are following this case very closely," Marshall Billingslea, a sleuth at the US Treasury department, told Berlingske.

"We have a close cooperation with the authorities here in Denmark, as well as in Estonia," the US official said.

The bank itself has declined to take a position on what happened, pending the publication, due later this month, of its internal probe.

"The matter is very complex, and no conclusion as to the number of suspicious customers or transactions - or indeed the extent of potential money laundering - can be drawn from any individual pieces of information taken out of context," a Danske Bank spokesman said on Monday, reacting to the Financial Times story.

"It's completely natural that there is interest for the case in Estonia from different authorities, including the US," another bank spokesman told the Reuters news agency.

"We take the matter very seriously ... [and] we are committed to understanding the full picture," Ole Andersen, the bank's chairman, said.