Toronto mayoral candidate Adam Giambrone has now admitted to 'multiple' affairs with women other than his partner. (CBC)

Toronto mayoral candidate George Smitherman is calling on his former rival for the city's top job, embattled councillor Adam Giambrone, to step down as chair of the Toronto Transit Commission.

Giambrone announced earlier Wednesday he is ending his mayoral campaign after admitted to having numerous intimate relationships with women other than his live-in partner of several years, Sarah McQuarrie.

Smitherman — who left his job as Ontario energy and infrastructure minister to run in the race to replace current Mayor David Miller — issued a statement Wednesday afternoon saying Giambrone's withdrawal "is not news that any of us greet happily.

"Nevertheless, under the circumstances, his decision is understandable and appropriate."

Giambrone, 32, will return to his duties as councillor and TTC chair starting next week, said the councillor's assistant Kevin Beaulieu.

But Smitherman said in his statement that the transit commission needed a leader who was "100 per cent focused" on improving service.

"It is clear that Mr. Giambrone is … not in a position to commit that degree of focus nor provide the leadership that is necessary to get the job done for riders, workers or taxpayers. With those interests in mind, he should step down as Chair of the TTC," Smitherman said in his statement.

Giambrone didn't make the announcement he was quitting the race himself — even though he held a news conference at Toronto City Hall on Wednesday morning to speak about his personal life.

Instead, several minutes after Giambrone left the stage, Beaulieu walked before the cameras, re-read Giambrone's statement and then added his boss's run for mayor ends "as of today."

George Smitherman says Giambrone's decision to quit the race was 'understandable and appropriate.' (Canadian Press)

Mayor, Moscoe offer support

Reaction from city council to the announcement, meanwhile, was swift.

Miller released a statement declaring his support for Giambrone, calling his decision to end his campaign regrettable, but adding "his decision to focus on his private life and his current duties is understandable."

Coun. Howard Moscoe, meanwhile, told CBC News that Giambrone has "done a good job running the TTC.

"I don't think his personal life should affect that."

But Coun. Brian Ashton saw things differently.

"I don't believe he can [lead the TTC]. I think what he's been going through is an enormous distraction."

The trouble for Giambrone began on Tuesday, when the Toronto Star quoted Toronto university student Kristen Lucas as saying she had a sexual relationship with him. Giambrone admitted to the paper that he had an "inappropriate relationship" with Lucas, despite his years-long relationship with McQuarrie.

McQuarrie was introduced amid the pomp and glitz of Giambrone's campaign launch last week as his long-time partner. Friends say Giambrone and McQuarrie have been together for about five years.

On Wednesday, Giambrone was back on the front page of the Star, admitting he had affairs "with other women."