it was an "open secret" she was sleeping with Boris.
He became friends with Ms Arcuri, 34, during her time on the London tech scene - when she's alleged to have had a fling with the then-mayor.
Both the Prime Minister and Ms Arcuri - who appeared in a bombshell GMB interview yesterday - have refused to deny an affair.
But she and Boris, 55, have vehemently denied any impropriety in relation to Ms Arcuri's business being awarded £126,000 in taxpayer cash.
'LOUD AND PROUD'
Claiming he tried to warn her off sleeping with then-married Boris, Mr Yiannopoulos insisted Ms Arcuri would "loudly and proudly" display evidence of their alleged romance.
He said: "Jennifer and I were friends for a few years, as she schmoozed her way around the London tech scene.
"Throughout this period, Jennifer was bragging to me and others that she was having sex with Boris Johnson in her Shoreditch flat.
"I warned her many times about getting physically involved with Boris, but it was already too late. Their affair was well underway.
"Jennifer's sexual relationship with Boris won't be a surprise to anyone around during the heady days of the Tech City boom.
"It was an open secret - mostly because Jennifer loved to advertise it, especially after a few glasses of wine."
Jennifer showed up to a Farringdon steak restaurant in a jaw-droppingly short skirt, with plum-coloured marks on her thighs and breasts that she proudly - and loudly - described as 'Boris bruises
Milo Yiannopoulos
Mr Yiannopoulos, who appears in several cosy social media snaps with Ms Arcuri, sensationally claimed she flaunted bruises allegedly caused by her romping with BoJo.
He added: "Jennifer showed up to a Farringdon steak restaurant in a jaw-droppingly short skirt, with plum-coloured marks on her thighs and breasts that she proudly - and loudly - described as 'Boris bruises'.
"She said they were the result of sex with Boris. There was no suggestion that anything abusive had occurred.
"Rather, Jennifer flaunted the evidence of her affair, wearing them like trophies.
"They were the product of enthusiastic, consensual lovemaking, she said."
CASH PROBE
The cash awards given to Ms Arcuri's Innotech firm are now subject to Whitehall and City Hall probes.
She has previously explained away reports he regularly visited her East London flat - which had a dance pole - as being for "technology lessons."
And she insisted on Good Morning Britain on Monday that she received no extra advantage from her "friendship" with Boris, who she saved in her phone as Alex the Great - after Boris' given first name
She insisted there was no corruption in their relationship, saying: “Boris never ever gave me favouritism”.
'PROFESSIONAL ADVANTAGE'
But Mr Yiannopoulos claimed: "Jennifer frequently alluded to the professional advantage she might realise from her liaison.
"She even confided in me that Boris had written her a letter of recommendation for the Tech City position" - a £100,000-a-year to Government advisory role for which she was reportedly massively under-qualified.
A bitter Mr Yiannopoulos, who went on to become a fervent Trump supporter, added: "The Jennifer Arcuri I remember wanted, more than anything, simply to be famous.
"She was prepared to do anything to get there."
BOMBSHELL INTERVIEW
Ms Arcuri, who first met Boris in 2011, refused to deny six times that they were lovers for four years when she broke her silence on GMB yesterday.
Repeatedly quizzed on whether they had an affair, the ex-model would only say: “I’m not going to be putting myself in a position for you to weaponise my answer.
“It’s no one’s business what private life we had, or didn’t have”.
But Ms Arcuri did admit the pair, who first bonded over their mutual love for writers Voltaire and Shakespeare, “share a very close bond”, adding: “I care about him deeply as a friend”.
Ms Arcuri moved to California last year with British partner Matthew Hickey and their young daughter.
Labour has demanded the PM explain why he failed to declare the friendship, despite Ms Arcuri accompanying him on three foreign trips.
Asked whether he broke the Greater London Assembly code, the PM would only say yesterday: “No, and I’ve said everything I am going to say on that matter”.
Labour Shadow Chancellor John McDonnell said: “Regardless of the exact nature of his relationship with Arcuri, it is clear that she and Boris Johnson were close, and that he misled the public when he said ‘there was no interest to declare’.
“The Prime Minister is unfit for office.”
But independent Tory MP Ed Vaizey told BBC2’s Politics Live that the inquiries into Boris Johnson and businesswoman Jennifer Arcuri will “probably be helpful to Boris”.
He said: “I think one is slightly clutching at straws if you’re saying that somebody you get on with, you therefore need to declare an interest”.
Jennifer Arcuri gets angry with Piers Morgan as she denies relationship with Boris Johnson gained her an unfair place on trade trips
We pay for your stories! Do you have a story for The Sun Online news team? Email us at tips@the-sun.co.uk or call 0207 782 4368. You can WhatsApp us on 07810 791 502. We pay for videos too. Click here to upload yours.