Toronto Mayor Rob Ford has been admitted to a hospital and is believed to have a tumour in his abdomen, health officials said.
Dr Rueben Devlin, from Humber Regional Hospital in the city, said Mr Ford had been complaining of abdominal pains and that an examination had resulted in a working diagnosis of a tumour.
He said Mr Ford had been suffering from stomach pains for at least three months and they had worsened in the last 24 hours.
"It became unbearable for him," he added.
Mr Ford, 45, became an international celebrity last year after he acknowledged using crack in a "drunken stupor" following months of denials.
The mayor returned to work in June after a rehab stint for drug and alcohol abuse and is running for re-election on October 27.
Dr Devlin said they need to determine what type of tumour it is by doing a biopsy. He said he could not say how long Mr Ford will be in the hospital.
Dr Devlin said the CT scan is "very definitive for the tumour" but a "definitive diagnosis" is still to come.
The Toronto Sun reported that Mr Ford was taken to hospital in 2009 for a tumour on his appendix, leading to its removal along with part of his colon.
Doug Ford Sr, the mayor's farther, died of colon cancer in 2006.
Councillor Doug Ford, the mayor's brother and campaign manager, said Rob is in good spirits. Doug Ford sidestepped questions about the mayoral race.
Although campaigning has been under way for months, the official deadline for candidates to sign up is tomorrow. That leaves time for Doug Ford, a city councillor, to step in for his brother, a possibility some have long speculated about.
Doug Ford deflected a question about his brother's previous health issues. He said the mayor complained of stomach pain while the pair had breakfast.
"He said his stomach was bothering him. He went to the doctor's and the doctor sent him over to Humber," Doug Ford said.
The mayor has steadfastly refused to step down since reports emerged of a video of him smoking crack.
Olivia Chow, who is running to replace Mr Ford, tweeted that her thoughts and hopes are with him and his family for good news in the days ahead.
John Tory, considered the front runner in the race, wished Mr Ford a speedy recovery. "My thoughts are with Mayor Ford, Renata, their children and the entire Ford family this evening," Mr Tory said in a statement.