Michael Cohen, formerly a lawyer for President Trump - AP

Michael Cohen, Donald Trump’s former lawyer and fixer, is being investigated over $20 million of loans obtained by taxi companies owned by him and his family, according to The New York Times.

His close relationship with the president and the Trump Organisation have made him a pivotal figure in the slew of investigations into alleged collusion between the Trump campaign and Russia.

Federal prosecutors in New York are reportedly investigating him for possible bank and tax fraud, as well as a $130,000 payment to adult film star Stormy Daniels.

He has not been charged with any crime. He and his lawyers declined to comment when contacted by The New York Times.

Details of the size of the investigation come at a time when Mr Trump has revealed his fury at federal prosecutors picking their way through his team.

White House Counsel Donald McGahn has reportedly been cooperating extensively with Robert Mueller's inquiry Credit: AFP

Lashing out at reports that the White House's most senior legal adviser has been drawn into the inquiry, Mr Trump said the investigation into his campaign's links with Russia makes "Joseph McCarthy look like a baby” as he .

Sending an unusually long string of tweets, the president appeared to be rattled by earlier revelations in The New York Times that Don McGahn, White House counsel, has spent more than 30 hours talking to agents during voluntary interviews.

Mr McGahn is reported to have described the president’s anger at the Russia investigation and the ways in which he asked his legal team to respond.

That anger was on full display on Sunday, when Mr Trump compared Robert Mueller’s investigation with the 1950s hunt for communists led by Senator Joseph McCarthy, an episode that is remembered today as a dark period in America’s history.

“Study the late Joseph McCarthy, because we are now in a period with Mueller and his gang that make Joseph McCarthy look like a baby! Rigged Witch Hunt,” wrote Mr Trump.

White House Counsel Donald McGahn (L) and White House economic advisor Larry Kudlow (R) listen behind President Donald Trump Credit: Reuters

Last week, Mr Trump's critics accused him of McCarthyism as he stripped the security clearance of John Brennan, the former CIA director.

Story continues

The exchanges have become part of a regular weekend routine: Newspapers publish new details of Mr Mueller’s probe; the president fires off early morning Twitter broadsides; then his cheerleaders take to Sunday morning TV shows to fling back accusations of a witch hunt.

Study the late Joseph McCarthy, because we are now in period with Mueller and his gang that make Joseph McCarthy look like a baby! Rigged Witch Hunt! — Donald J. Trump (@realDonaldTrump) August 19, 2018

Rudy Giuliani, Mr Trump’s personal lawyer, appeared on NBC’s Meet the Press soon after Mr Trump's tweets, but the debate threatened to descend into farce as he tried to suggest that there was no such thing as objective truth.

The Failing New York Times wrote a story that made it seem like the White House Councel had TURNED on the President, when in fact it is just the opposite - & the two Fake reporters knew this. This is why the Fake News Media has become the Enemy of the People. So bad for America! — Donald J. Trump (@realDonaldTrump) August 19, 2018

Mr Giuliani pushed back against demands that Mr Trump himself testify before Mr Mueller’s team saying he could be sucked into a he-said, she-said squabble.

“When you tell me that, you know, he should testify because he’s going to tell the truth and he shouldn’t worry, well that’s so silly because it’s somebody’s version of the truth. Not the truth,” he said.

Chuck Todd, the host, responded: “Truth is truth.”

“No, it isn’t truth. Truth isn’t truth," countered Mr Giuliani, prompting a baffled Mr Todd to say that his words would soon become a "bad meme", an internet joke widely circulated on social media.

Rudy Giuliani: From America's mayor to defending Donald Trump

The same show provided the setting last year when Kellyanne Conway, a Trump adviser, used the term “alternative facts” to defend the White House press secretary’s false statement about attendance numbers at Mr Trump’s inauguration.

The revelations published on Saturday suggest Mr McGahn shared detailed accounts with investigators about the episodes at the heart of the inquiry into whether Mr Trump obstructed justice.

In six tweets, starting at 7:01am, Mr Trump denied that the White House counsel was now working with prosecutors.

“But I allowed him and all others to testify - I didn’t have to,” he wrote from his New Jersey golf club. “I have nothing to hide...... and have demanded transparency so that this Rigged and Disgusting Witch Hunt can come to a close.”

Did the Trump campaign collude with Russia in 2016? | The four investigations

At times, Mr Trump’s supporters have suggested the president could claim executive privilege or lawyer-client confidentiality to prevent such interviews. At other times, the White House has said it is in favour of co-operating in order to bring the investigation to a rapid close.

William Burck, an attorney for Mr McGahn, said in a statement: "President Trump, through counsel, declined to assert any privilege over Mr McGahn's testimony, so Mr McGahn answered the Special Counsel team's questions fulsomely and honestly, as any person interviewed by federal investigators must."

Mr Giuliani said in his television interview on Sunday morning: “The president encouraged him to testify, is happy that he did, is quite secure that there is nothing in the testimony that will hurt the president."