The 39-year-old EastEnders actress, who plays Kat Moon in the show, allegedly went "berserk" after discovering that her husband-to-be Vince Morse had been sending sordid emails to his ex-partner.
The 50-year-old catering boss emailed Karen Short on various occasions, according to claims made by The Sun. The paper also alleges that Morse told her in July that he had finished his relationship with Wallace, writing: "As of a week ago it's really been off but I'm only just admitting it to myself."
A close friend of Wallace has now said: "To say Vince is in the doghouse is a huge understatement. She went absolutely berserk and launched at him, then called it all off. She desperately loves him but just feels very, very let down and humiliated. She has told him that she will decide this morning whether to go ahead."
The couple's booking for Pembroke Lodge in Richmond for the wedding has not yet been cancelled.
Wallace has previously said of Morse: "Vince has got everything I've been looking for. I knew literally straight away I wanted to spend the rest of my life with him. I do believe there's one person out there for you and the more time we spend together the closer we become. It's like we're one person."
Earlier this month, Wallace ended a long-running feud with her sister Danielle. The pair became estranged in 2008 when the star was accused of stealing Danielle's boyfriend, but they have now decided to move forward for the sake of Wallace's daughter Tallulah and Danielle's new baby Rudy.