Mayoral candidate Adam Giambrone admitted Monday night he had "an inappropriate relationship with a young woman."

Calling it "a serious lapse in judgment," the Toronto city councillor offered his "deepest apology to my partner, Sarah, my family and friends for the pain and embarrassment my indiscretion will cause them."

His statement came after the Toronto Star asked him a series of questions about his relationship with university student Kristen Lucas, who said she had an intimate relationship with Giambrone for more than a year. Lucas, now 20, was 19 when she began dating Giambrone.

She told the Star Giambrone had told her his live-in partner of many years – who's been at his side for public events during his fledgling mayoral bid – is there for "political" reasons.

"You know I will be announcing I have a partner," Giambrone, 32, wrote in a text message to Lucas Dec. 27. "It is someone named Sarah (McQuarrie), who I've been involved with in the past. It is important for the campaign."

Giambrone told Lucas he hoped they could continue to see each other, assuring her: "I had to have someone political."

In recent interviews with the Star, Lucas said she's been involved with Giambrone since late 2008 and, on several occasions, had sex late at night on a couch in his City Hall office.

Giambrone, who launched his first bid for the mayor's chair Feb. 1, is also chair of the troubled Toronto Transit Commission.

In a statement Monday night to the Star, Giambrone said the relationship with Lucas "consisted of text messages and conversations in public places only. I met her mother, having dropped her off at home."

Further, Giambrone wrote; "Realizing the mistake this relationship was I tried to end it some months ago and finally broke off all contact."

Lucas provided the Star with what she said were text messages between the pair dating back to last year, as well as Facebook correspondence.

She alleged Giambrone revealed secrets relating to his work – "I knew a long, long time ago about the (TTC) fare hike" – and shared his "negative" opinions of several councillors. She offered no details, but said Mayor "David Miller is like a god to Adam."

Lucas said she told only her mother, non-profit social worker Lori Lucas, about the TTC hike, which was announced in mid-November and took effect Jan. 1. Her mother said she kept the secret.

Curled in an armchair in her mother's living room Sunday, Lucas, slight with curly black hair, said she was shocked in December to learn about McQuarrie, whom Giambrone describes publicly as his "longtime live-in partner."

McQuarrie, wearing pearls and narrow glasses, strode onto the stage after Giambrone's speech at his campaign launch last week, doing air pumps with him before descending into the crowd for a joint meet-and-greet with supporters.

"I really wanted to tell you (about McQuarrie) when we met," Giambrone writes Dec. 27, 2009, according to text messages taken from Lucas's phone. "I hope this does not change things.

"We're still going to meet like we said next week, right?" he asks.

Replies Lucas: "No, it changes everything. Why couldn't it have been me?"

He insists he had to have someone "political," and asks: "Do you mean we can't talk?"

In another text message, Giambrone tells her: "I still think of you when I need ... um ... stimulation."

Further: "I like you because you're smart and interesting. You're also good-looking naked."

Lucas insisted Giambrone "told me he didn't have a girlfriend."

Although he never took her to his apartment, she believed he was concerned his elderly parents, who lived nearby, might drop by unannounced.

"I am actually pretty transparent," he told her, in an early Facebook message, Dec. 21, 2008. "I live with my brother. I swim a lot and like to spend time travelling. How's that for a beginning."

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A year later, she reacted angrily to the news about McQuarrie, texting: "Please don't tell me you're doing this for the campaign. You're actually going to build a life you don't want? That's mind-blowing.

"You HAD to choose someone good for your campaign. That's f------ ridiculous."

Later Giambrone insists, "I am NOT marrying (Sarah)."

According to Lucas, they had sex for the last time Jan. 2, this year, in her bedroom. Later, lying in bed and talking, she says she agreed to keep seeing him – provided he told McQuarrie about their relationship.

Earlier that evening, Giambrone had attended a function with McQuarrie at Wanza, an Ethiopian restaurant on the Danforth, where the couple was photographed about to sample his favourite dish of onions, red peppers and spare ribs.

Lucas, who'd written she'd "wear something nice," waited at her place.

When she messaged him to ask where he was, Giambrone replied: "Yes, still here at the restaurant, see the photo in the Sun tomorrow."

Later, a text says he's en route to her home, first 15 minutes away, then five.

Monday night, Giambrone said in his statement Lucas had "earlier sent an email I thought was threatening."

His campaign sent the Star a copy of an email that it claimed had been sent to Giambrone by Lucas on Jan. 7.

In the email she threatens to track down "ur `girlfriend' and let her know about the affair we're having. It doesn't have to be true ... ha! I hope U become mayor so that I could become TTC chair!"

Lucas denied sending the email and pointed out her name is spelled incorrectly.

She said Giambrone called her Saturday morning and kept telling her he was sorry. He also insisted he and Lucas had always been just friends, she said.

Each time she challenged a statement as mistaken, he backed down and apologized again, she said.

Before they hung up, she said she asked if he was sleeping with McQuarrie while he was sleeping with her. According to her, he said he had been.

In his statement Monday night, Giambrone said: "I called her on this past weekend to ask her not to reveal the relationship."