Furious Gabriela Irimia is refusing to see the Lib-Dem politician or take his calls after a series of bust-ups.
Last night a source close to the pair said: "Gabby is very stressed and just wants to be on her own.
"She is very unhappy and has been crying a lot. It's just got too much lately."
Gabby-whose most famous hit with twin Monica is The Cheeky Song (Touch My Bum)-has told friends: "I can't handle it any more. This is not a proper relationship. I want some space."
Friends say a series of rows came to a head two weeks ago, before the Cheeky Girls performed at a gala dinner in London.
Our source revealed: "Lembit was going on and on, dominating the conversation like he normally does and was delaying Gabriela, who wanted to get ready to sing with Monica. She was going to be late on stage and decided she'd had enough.
"She stormed off, did the gig and refused to speak with Lembit at the end or leave with him.
"Lembit has been frantically texting her but she doesn't want to see him or talk to him.
"He even turned up at the twins' home in Rye last week but Gabby refused to see him."
The relationship was regarded as one of the oddest celebrity pairings ever-he a 42-year-old politician and she a Romanian pop star 17 years his junior.
They met when taking part in C5's All Star Talent Show in 2006 and got engaged three months ago when Opik went down on one knee by the Trevi Fountain in Rome. The couple discussed marrying next year though they never lived together, spending just two nights a week with each other at his London flat or in Wales where he owns two houses.
Opik was previously engaged to TV weathergirl Sian Lloyd. They had an acrimonious split, with her criticising his drinking.
Tensions in his relationship with Gabby were clear in documentary Living With The Cheeky Girls, repeated on Living TV tonight at 8pm.
In it, the couple are seen arguing after Opik arrives late for a party to celebrate the 53rd birthday of the twins' mum Margit.
He had been held up voting at Westminster and missed a speech by Gabby at London's Embassy Club. As he explains what happened, Gabby walks off saying "whatever".
Another source claimed: "Lembit dictates when and where they meet and Gabby's finding it emotionally draining.
"They met accidentally on Thursday at the BBC during filming of political programme This Week.
"He turned up and asked what Gabby and Monica were doing there. Then his researcher made a joke in front of everyone, asking the same question. The twins felt insulted. You could have cut the atmosphere with a knife.
"Lembit tried to buttonhole Gabby in the green room but she was ice cold. She left with Monica, refusing to say goodbye."
Last night mum Margit said: "Gabriela is upset but is refusing to talk about Lembit."