A biographer of Donald Trump who was sued for $5bn (£3.9bn) by the president - unsuccessfully - over accusations of lying said he was threatened with supposed tapes of the pair's conversations in an effort to intimidate the journalist.

Tim O'Brien, who wrote TrumpNation: The Art of Being the Donald, published in 2005, told CNN's Reliable Sources that Trump repeatedly told him he was recording their conversations.

But when Trump took O'Brien to court in the mid-2000s because his book cast doubt on a number of claims by the property magnate - most significantly, that his net worth ran into the billions, when evidence suggested it may only be as much as $250m - Trump was forced to admit he had no tapes.

Moreover, the lawsuit exposed Trump to intense scrutiny and he had to backtrack on a whole number of outlandish claims, including one that he had received no financial help from his father to set up in business. In fact, Trump had been handed $9m, which he was forced to admit. Trump was also caught out on how much he really earned from speaking fees and the sales of his luxury condos.

"His loose association or relationship with the truth becomes problematic for him when he's confronted with documents that are contrary to things that he's said publicly," O'Brien told CNN. The journalist's attorney in the lawsuit asked Trump why he had lied about having the interview tapes in the first place. "He essentially said: 'I wanted to intimidate him'," O'Brien said.

Trump has recently used the threat of releasing tape recordings in relation to his conversations with the former FBI director James Comey, who the president fired ostensibly for reasons of competence. Trump's critics claim the sacking was an effort to disrupt an ongoing FBI probe into links between his presidential campaign and Russia.

The president is coming under pressure to release the purported tapes after Comey accused him in front of the Senate of telling "lies, plain and simple" about the content of their private conversations.

Donald Trump
President Trump once tried to intimidate a journalist by claiming to have recorded their conversations. Miguel Medina/AFP/Getty Images