Donald Trump’s personal lawyer has taken responsibility for a tweet in which the US President seemingly admitted he knew his security adviser Michael Flynn lied to the FBI before he was fired in February.

The tweet implied Mr Trump knew Mr Flynn had committed a felony by lying to the FBI when he told the agency’s then-director James Comey to go easy on Mr Flynn the day after the sacking.

Interfering in the FBI’s investigation into Mr Flynn could be construed as obstructing justice and a potential trigger for the President’s impeachment.

I had to fire General Flynn because he lied to the Vice President and the FBI. He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!

— Donald J. Trump (@realDonaldTrump) December 2, 2017

Mr Trump’s personal lawyer, John Dowd, raised eyebrows Monday morning (AEST) when he claimed to have written the tweet, not the President.

In an interview with Axios, Mr Dowd said the tweet was “my mistake”. 

“I’m out of the tweeting business,” he said. “I did not mean to break news.”

Mr Trump later slammed the FBI in a stunning Twitter tirade in which he  described the law enforcement agency’s reputation as the “worst in history”.

“After years of Comey, with the phony and dishonest Clinton investigation (and more), running the FBI, its reputation is in Tatters — worst in History!” Mr Trump tweeted on Sunday.

“But fear not, we will bring it back to greatness.”

After years of Comey, with the phony and dishonest Clinton investigation (and more), running the FBI, its reputation is in Tatters – worst in History! But fear not, we will bring it back to greatness.

— Donald J. Trump (@realDonaldTrump) December 3, 2017

The agent had also worked on the investigation of Hillary Clinton’s use of a private email server.

Mr Trump took advantage of the officer’s dismissal, tweeting: “Report: ‘ANTI-TRUMP FBI AGENT LED CLINTON EMAIL PROBE’ Now it all starts to make sense!”

“Tainted (no, very dishonest?) FBI ‘agent’s role in Clinton probe under review.’ Led Clinton Email probe,” he added.

Mr Trump also retweeted a tweet suggesting FBI director Chris Wray “needs to clean house”.

The President said earlier Sunday he never asked former FBI director James Comey to stop investigating ex-national security adviser Michael Flynn.

“Just more fake news covering another Comey lie!” he tweeted.

Mr Flynn, who was also a former aide in Mr Trump’s election campaign and a central figure in a federal investigation into Moscow’s alleged interference in the 2016 US presidential election, pleaded guilty on Friday to lying to the FBI.

Former national security advisor Michael Flynn leaves court after his guilty plea. Photo: Getty

Mr Flynn pleaded guilty to obstructing justice after lying about conversations he had with Russian ambassador Sergey Kislyak in 2016.

According to court documents, a member of Mr Trump’s presidential transition team, later identified by sources as Mr Trump’s son-in-law Jared Kushner, told Flynn on December 22 to contact officials from foreign governments, including Russia, to convince them to delay the vote or veto the resolution.

Mr Flynn spoke with then-Russian Ambassador to the United States Sergei Kislyak that day, and again the following day, according to the court documents.

Mr Flynn also filed materially false statements and omissions in his March 7, 2017 foreign agent filing over his company’s work with the Turkish government, according to prosecutors.

His plea agreement, and his decision to cooperate with the investigation led by Mr Mueller, marked a major escalation in a probe that has dogged Mr Trump’s administration since the Republican president took office in January.

Mr Flynn acknowledged making false statements about contacts he had with Russia’s ambassador to the United States, Sergei Kislyak, last year.

The charges carry sentence of up to five years in prison.

Mr Flynn was fired from his White House post in February for misleading Vice President Mike Pence about his conversations with the ambassador.

– With agencies