Donald Trump’s longtime confidant and former lawyer Michael Cohen poses a radical new threat to the US president, claiming Mr Trump knew in advance of a 2016 meeting at Trump Tower with Russian representatives.

Mr Cohen’s claims directly contradict those of Mr Trump who again on Friday denied he knew about a meeting between his son Donald Trump Jr and a group of Russian officials who offered to provide damaging information about Democratic rival Hillary Clinton.

According to a CNN report, Mr Cohen is willing to make his assertion to special counsel Robert Mueller. Mr Cohen claims he was present when Mr Trump was informed of the offer by his son and according to Mr Cohen, approved going ahead with the meeting.

Mr Mueller is investigating whether Mr Trump’s campaign worked with Russia to influence the outcome of the 2016 presidential election.

Mr Cohen was considered one of Mr Trump’s most loyal confidants but the relationship between the men has soured following an April FBI raid on Mr Cohen’s office and home.

Mr Cohen previously insisted he would remain loyal to the president – even once claiming he was “take a bullet” for Mr Trump – but in an interview with ABC News earlier this month, he signalled his willingness to cooperate with Mr Mueller.

“My wife, my daughter and my son have my first loyalty and always will,” he said. “I put family and country first.”

Investigators are trying to establish whether Mr Cohen paid women who claimed to have damaging information about then-presidential candidate Donald Trump hush money in exchange for their silence, a violation of federal campaign laws.

In a series of tweet on Friday, the US president claimed Mr Cohen was fabricating stories in order to escape being charged with possible tax fraud.

“I did NOT know of the meeting with my son, Don jr,” Mr Trump said in a tweet after the CNN report was published.

In another tweet, Mr Trump called Mr Mueller’s investigation a “rigged witch hunt”.

Arrived back in Washington last night from a very emotional reopening of a major U.S. Steel plant in Granite City, Illinois, only to be greeted with the ridiculous news that the highly conflicted Robert Mueller and his gang of 13 Angry Democrats obviously cannot find Collusion…

— Donald J. Trump (@realDonaldTrump) July 27, 2018

….,the only Collusion with Russia was with the Democrats, so now they are looking at my Tweets (along with 53 million other people) – the rigged Witch Hunt continues! How stupid and unfair to our Country….And so the Fake News doesn’t waste my time with dumb questions, NO,….

— Donald J. Trump (@realDonaldTrump) July 27, 2018

…..I did NOT know of the meeting with my son, Don jr. Sounds to me like someone is trying to make up stories in order to get himself out of an unrelated jam (Taxi cabs maybe?). He even retained Bill and Crooked Hillary’s lawyer. Gee, I wonder if they helped him make the choice!

— Donald J. Trump (@realDonaldTrump) July 27, 2018

The June 2016 meeting was arranged between Donald Trump Jr and a publicist who told him a senior Russian official was offering to provide the Trump campaign with damaging information about Hillary Clinton as part of the Russian government’s “support for Mr Trump”.

Mr Trump’s son-in-law Jared Kushner and campaign chairman Paul Manafort were also present at the meeting at Trump Tower in Manhattan.

Putin invites Trump to Moscow

The stoush comes as Mr Trump looks certain to have another meeting with Russian President Vladimir Putin.

Speaking to reporters while in South Africa on Friday, Mr Putin said he had invited the US leader to Moscow for further talks on issues like the Iran nuclear deal.

“Regarding our meetings, I understand very well what President Trump said. He has a desire to have further meetings,” Mr Putin said. “I am ready for that.”

“We need for the appropriate conditions to exist, to be created, including in our countries.”

In a statement, the White House said Mr Trump would accept Mr Putin’s invitation. The pair met in Helsinki last week in a two-hour closed-door meeting which sparked bipartisan outrage in Washington.

Mr Putin was due to visit the United States later this year but the visit was postponed until 2019 amid fierce condemnation from both Republican and Democratic stalwarts.

Ebony Bowden contributed reporting from New York City.