TORONTO (Reuters) - Toronto Mayor Rob Ford, who gained global notoriety after a well-publicized battle with addiction, was admitted to hospital with a tumor in his abdomen on Wednesday, throwing his campaign for re-election into doubt.

Ford went to a Toronto hospital, complaining of abdominal pains that had lasted three months and worsened over the past day, a hospital official told reporters.

"Examination and investigation today revealed that he has a tumor," Dr. Rueben Devlin, the hospital's president said at a news conference in the north Toronto facility. "It's not a small tumor (but) the size is not as relevant as what it is.

"It's been going on for greater than three months, but today it became unbearable for him."

Devlin said tests to determine the nature of the tumor will be done this week, including a biopsy to see if the tumor is malignant and whether it has spread.

Ford became famous after admitting in November that he had smoked crack cocaine while in a "drunken stupor" and had driven after drinking. He entered a rehabilitation facility in May. Throughout the ensuing controversy, he said he would stand for re-election and has campaigned energetically.

Doug Ford, the mayor's brother and campaign manager who is also a city councillor, said the mayor complained about abdominal pains over breakfast. After seeing a doctor, he was quickly admitted to hospital.

A visibly shaken Doug Ford refused to comment on whether his younger brother would suspend his mayoral campaign or drop out.

"It saddens me that I have to be here today. Rob's in good spirits and just wanted to thank the well-wishers for all the calls that are coming in," Doug Ford said, asking for privacy for his family. "We'll speak about (the campaign) tomorrow."

Rob Ford, who participated in a mayoral debate and met with boxer Mike Tyson at City Hall on Tuesday, had a tumor on his appendix removed in 2009, according to media reports. Ford's politician father died of colon cancer in 2006.

The mayor, who came to power pledging to cut waste at city hall and keep a lid on taxes, has a core base of suburban support in Canada's largest city. But he has trailed in the polls ahead of the Oct. 27 election. A new poll published on Wednesday showed Ford running second behind former businessman John Tory, with 28 percent support compared with 40 percent for Tory. Left-leaning candidate Olivia Chow was in third place in the three-way race.

"My thoughts and hopes are with @TOMayorFord and his family for good news in the days ahead," Chow tweeted on Wednesday night.

Tory also sent a message of support to his political rival.

"While you may see us debating, we are also human beings," Tory said in a statement, adding he wanted Ford "back where he would want to be - with us at the debating tables talking about the city we all love."

(Reporting by Amran Abocar; Editing by Ken Wills)