Nick Candy has told a court he regretted ever deciding to become a friend of a man now suing him for millions of pounds, saying it was "the worst decision of my life by a long, long way".
Mr Candy and his brother Christian deny allegations by Mark Holyoake that they conspired to extort large sums from him by putting "the fear of God" into him with threats.
Mark Holyoake is claiming £132 million in damages against the property tycoons at the High Court in London.
But Nick Candy told a judge in London the "very, very charismatic" Mr Holyoake was "a fraudster and pathological liar".
Mr Holyoake was Nick's long-time friend from university days until things turned sour over a £12 million loan from the brothers.
Mr Candy said they had become "close friends" but Mr Holyoake was not his best friend.
He said: "I regret the day I ever met the man.
"It was the worst decision of my life by a long, long way."
Mr Holyoake alleges the Candys made threats against both him and his family to extort repayments amounting to £37 million.
The brothers, who are best known for creating the luxury One Hyde Park apartment complex in Knightsbridge, central London, deny all the allegations.
Mr Holyoake borrowed the money from Christian Candy's company CPC Group in October 2011 to help him buy Grosvenor Gardens House in Belgravia, London, to convert into a luxury property for a substantial profit.
He gave evidence earlier in the trial that he eventually paid back more than £37 million on the two-year loan and had to sell up at a loss of £100 million in potential profits after incurring millions of pounds in legal fees and other costs.
Mr Holyoake, 44, alleges the brothers subjected him and his family to a campaign of intimidation, including threats against his then pregnant wife Emma.
They had put "the fear of God" into him by threatening to sell his debt to Russian gangsters, it was alleged.
Earlier in the trial Christian Candy told Mr Justice Nugee from the witness box: "I don't deal with shady people."
Nick Candy told the court in cross examination how from the early days he and his brother made "tens and tens of millions" from their property development enterprises as prices boomed in London.
Asked by Roger Stewart QC, representing Mr Holyoake, if he was tough in business, Mr Candy replied: "I am no pussycat, that's for sure."
He told the judge he could be tough but was "fair", then added: "My wife will tell you I am an emotional softie."
Questioned about his relationship with Mr Holyoake, Mr Candy told the court what made him "sick about this case" was the allegation "that I threatened his family".
He added: "It makes me absolutely sick.
"I have got a three-year-old child.
"If anyone threatened my family I would be absolutely appalled by it."
Mr Candy told the court he "came from nothing".
He agreed Katy Perry was paid around £1.2 million to sing at his wedding to his actress wife Holly Valance in 2012
He said marrying the former Neighbours star was a "one-off event" .
"I am only planning on getting married once, hopefully."
Mr Candy described in a written statement how he first met Mr Holyoake during their first year at Reading University in the early 1990s, sharing a house with six others during their second year and becoming good friends at the time.
Mr Candy said Mr Holyoake then went to France as part of his degree course and the pair were not in touch for about 10 years, then they became close friends once more.
"Our relationship was, however, purely social and we did not do any business together.
"Mr Holyoake did occasionally ask me to attend meetings with him (perhaps to bolster his credibility) but they did not involve business on my part."
Mr Candy said: "The entire situation with Mr Holyoake had been so emotional and painful for me."
He described doing his "very best to help out a friend" by introducing Mr Holyoake to CPC, and Christian, who loaned him "a significant sum of money on very short notice" to buy Grosvenor Gardens House.
Mr Holyoake "repaid me, Christian and CPC by continually lying to us every step of the way", the witness said.
The court was told earlier the Candy defence team had uncovered documents related to the collapse of Mr Holyoake's fish import business, British Seafood.
They allegedly show Mr Holyoake told the investment firm 3i, one of the British Seafood creditors, that he had net assets of £15 million in December 2010.
Less than a year later, a declaration of assets given to Christian Candy's Guernsey-based CPC group, as part of the Holyoake loan contract, had put Mr Holyoake's worth at the much larger figure of £120 million.
Candy lawyers say this later declaration did not take account of money promised to 3i and amounts to serious fraud.
Nick Candy said in his statement to the court on Friday that, in the wake of the British Seafood revelations, he had sent an email to his brother saying: "There is no doubt in my mind that (Mr Holyoake) is a fraudster and all I care about is getting CPC's money back."
He had also sent another email telling him: "Remember Mr Holyoake is a pathological liar".
Mr Candy, who described himself as a "networker and facilitator", said during the period the Holyoake loan was in existence he was primarily focused on his own business, C&C, and his own life, including supporting his then fiancee Valance as she took part in Strictly Come Dancing in 2011.
His projects included the final marketing phase and opening of One Hyde Park and a number of design projects for C&C clients in the UK and overseas.