Mikayla Bayliss, 23, told the Sun's Bizarre column she was dating Jamie for a year and he spent every night at her's including during the time he started a romance with Amber.
The model claimed he ditched her as his plus one to a wedding to take Amber even though he had slept with the blonde beauty the night before.
She said: "He hadn't stopped seeing me, we were always in contact everyday. There wasn't a point that we were not together.
"I slept with him the day before he took Amber to a wedding on May 7."
The Sun revealed Amber's romance with the hunk earlier this week and a source said the pair were making long term plans for the future.
The lothario was allegedly still staying round his girlfriends flat until May 4 even though he had just bagged the Towie star.
She added: "I think he’s been sleeping with us both 100 per cent.
"The minute I found out about Amber I rang and spoke to him and then I kind of just realised that I don’t need him in my life."
When Mikayla confronted Jamie he denied cheating and allegedly called her a "s***", a "nasty person" and accused her of being the cheat.
She added: "He wouldn’t admit to it when I confronted him and turned it round on me saying that I’m a slag and a nasty person and that I deserve this.
"It knocked me for ten. I didn’t know how he could say such nasty things to somebody he said he was in love with two days ago."
The heartbroken model says she told Amber of Jamie's cheating ways by messages and calling her.
Mikyala said: "She just hung up on me.
"Then I reached out to her again and said, ‘look babe, seriously this guy is my boyfriend, what’s been going on?’
"She’s just ignored it all."
She told the Sun that the guy she fell in love with was "sick" for his shocking two timing ways.
Mikayla added: "Looking back at my relationship I was in love with this guy who would just like use me.
"I think he’s getting kicks out of the fact that she was on Towie.
"He’s cheated on me with her and either knew or didn't.
"It’s sick, why would you want a guy who’s been with his ex and cheated on you."
Amber's representatives have declined to comment on Mikayla's claims.
Jamie Enver was contacted for comment.
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