Tension began when the Loose Women star playfully pulled at Karissa's trousers as the girls danced to a welcome burst of music last night.
The Playboy model told Welch afterwards that she was "very p*ssed" at the prank and ordered her to go away, later ranting to Frankie Cocozza in the garden: "[Denise] knows I'm a reserved person. I don't walk around flashing my t*ts or showing my ass. It's not funny."
McLean attempted to diffuse the row by keeping Karissa out in the garden while she talked to Welch, but when the 53-year-old continued to pick at the American housemate, she snapped: "Do me a favour, if you want it to kick off then I'll let her back in!"
While Cocozza tried to get the girls to stop shouting - being dismissed as "an 18-year-old boy" by Karissa in the process - McLean took the opportunity to share her disapproval of Welch's topless jacuzzi session last week.
"I was a page three girl for a lot of years, and you flash your boobs openly on telly and you're cool with that?" she asked. "I'm not!"
Welch angrily responded: "I've had a lot of things I've wanted to say to you in the house!"
The argument then saw the ladies go head-to-head over the recent shredder task, in which McLean destroyed messages from home addressed to Welch and four other housemates so she could win her own.
"I'm not comfortable with showing my body on telly!" McLean blasted, prompting the Benidorm actress to bite back: "Well I'm not comfortable with you pressing the buzzer without consulting anyone else whether they want their letters from home or not!"
The fight wound down with the intervention of Gareth Thomas and Romeo Dunn. The Welsh rugby player confessed afterwards that he had been upset by the night's events.
> Big Brother: Denise Welch loved ones 'concerned for her health'
Watch the beginning of the Celebrity Big Brother row below: