MSNBC host Lawrence O'Donnell said a "source close to Deutsche Bank" told him that President Donald Trump's loans were underwritten by "Russian billionaires close to Vladimir Putin."

"If true, that would explain every kind word Donald Trump has ever said about Russia and Vladimir Putin," O'Donnell said.

There has been no other reporting that substantiates this claim.

On Wednesday O'Donnell tweeted that he "made an error in judgment by reporting an item about the president's finances that didn't go through our rigorous verification and standards process. I shouldn't have reported it and I was wrong to discuss it on the air."

"I want to stress that is a single source, that has not been confirmed by NBC News, I have not seen any documentation from Deutsche Bank that supports this and verifies this... it's going to require a lot more verification before that can be a confirmable fact," said O'Donnell at the end of his show Tuesday.

Earlier on Tuesday, Deutsche Bank confirmed that it held tax records to do with Trump, but no details have yet been made public.

Trump's ties to Deutsche Bank have long been the subject of rumor and speculation, and O'Donnell has a long record of controversial statements.

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MSNBC host Lawrence O'Donnell added to the swirl of rumor that surrounds President Donald Trump's relationship with Deutsche Bank, suggesting that his loans with the lender were underwritten by Russian billionaires.

On Wednesday, O'Donnell expressed regret for reporting the unverified claim, tweeting: "Last night I made an error in judgment by reporting an item about the president's finances that didn't go through our rigorous verification and standards process. I shouldn't have reported it and I was wrong to discuss it on the air. I will address the issue on my show tonight."

White House press secretary Stephanie Grisham blasted O'Donnell's reporting in a statement to Fox News.

"This is one of the reasons that a majority of Americans have lost trust in the media," Grisham said in the statement, adding that journalists "and left-wing outlets have weaponized the media, using it to attack and harass people with little to no regard for the truth."

Trump's attorney Charles Harder also pushed back on the story, sending a letter to NBC (as well as Business Insider) demanding a retraction.

"These statements are false and defamatory, and extremely damaging," Harder wrote.

O'Donnell, appearing on the network Tuesday night with host Rachel Maddow, said that "this single source close to Deutsche Bank has told me that the Trump — Donald Trump's loan documents there show that he has co-signers. That's how he was able to obtain those loans. And that the co-signers are Russian oligarchs."

Later in the show he repeated the claim, saying: "The source close to Deutsche Bank says that the co-signers of Donald Trump's Deutsche Bank loans are Russian billionaires close to Vladimir Putin."

"If true, that would explain every kind word Donald Trump has ever said about Russia and Vladimir Putin," he said.

"If true, that would be a significant factor in Vladimir Putin's publicly stated preference for presidential candidate Donald Trump over presidential candidate Hillary Clinton."

O'Donnell, an outspoken critic of Trump, did not claim to have been able to corroborate the source's claim and no other news organisations have substantiated the claim.

O'Donnell did not reveal his sources identity, or the nature of the sources connections to Deutsche Bank.

Deutsche Bank did not immediately respond to a request for comment.

A spokesperson for MSNBC referred Business Insider to comments O'Donnell made towards the end of his show Tuesday, stressing that his claims were based on his own reporting rather than that of NBC News.

"I want to stress that is a single source, that has not been confirmed by NBC News, I have not seen any documentation from Deutsche Bank that supports this and verifies this. This is just a single source who has revealed that to me. And that where it stands at this point, its going to require a lot more verification before that can be a confirmable fact," said O'Donnell.

—Michael Del Moro (@MikeDelMoro) August 28, 2019

MSNBC has also faced criticism for missteps in its coverage of Trump's Russia ties in the past.

But as the president continues to hold back his tax records and other financial records from public scrutiny, they remain the focus for rumour.

The spotlight had again again fallen on Trump's ties with Deutsche Bank earlier Tuesday, when in a court statement it confirmed that it holds tax returns associated with Trump, his family or businesses.

Democrats have long sought information on Trump's financial affairs from the bank.

The Trump administration, though, is battling to block a subpoena issued by House Democrats for the president's financial records with the bank.

Officials argue that the Democrats don't have good enough evidence to warrant exposing the president's private financial affairs.

Also adding to speculation is Deutsche Bank's longstanding entanglements with money from the ex-Soviet Union.

The bank faces legal action, and its executives could be prosecuted, over the laundering of $20 billion from Russian criminals linked to the Kremlin and state security services.

(This article has been updated to reflect O'Donnell's tweet that he "shouldn't have reported" the unvetted story.)