Caroline Kennedy has withdrawn from consideration for the vacant Senate seat in New York, according to a person told of her decision.

Ms. Kennedy on Wednesday called Gov. David A. Paterson, who will choose a successor to Senator Hillary Rodham Clinton, to inform him that she no longer wished to be considered.

The person told of her decision said that Ms. Kennedy’s concerns about the health of her uncle, Senator Edward M. Kennedy, who suffers from brain cancer and was hospitalized after suffering a seizure Tuesday, prompted her to withdraw.

Ms. Kennedy believed that the job was hers if she would accept it, the person said, but aides to Mr. Paterson would not comment on whether that was true.

The news jolted the state’s political establishment and appeared to throw the Paterson administration into confusion.

Just before 7 p.m. Wednesday, Errol Cockfield, Mr. Paterson’s press secretary, said the governor had referred to the assertion that Ms. Kennedy was withdrawing as “just the rumor of the day.”

More than an hour later, Mr. Cockfield asked that that statement not be published, but neither he nor the governor’s communications director, Risa B. Heller, would respond to questions about Ms. Kennedy.

Ms. Kennedy’s decision comes nearly two months after she, along with several members of Congress and leading political officials, began auditioning to replace Mrs. Clinton in the coveted position.

She attracted relentless attention and was viewed by many as the most likely choice for Mr. Paterson, given her national stature and ties to the incoming Obama administration.

“We’re back to square one again,” said Doug Muzzio, a professor of Public Affairs at Baruch College. “It’s like Chutes and Ladders — we keep climbing and then we’re down the chute again.”

Ms. Kennedy had gained the support of some powerful backers in the state, including several labor officials and a top aide to Mayor Michael R. Bloomberg, Kevin Sheekey.

But her pursuit of the seat also set off resistance, with some local Democratic officials suggesting it smacked of entitlement, and polls showing voters preferring Attorney General Andrew M. Cuomo for the position.

Ms. Kennedy, 51, the daughter of President John F. Kennedy and a resident of the Upper East Side of Manhattan, has never served in public office.

Ms. Kennedy also stumbled initially with the news media, refusing at first to answer questions about why she was running, but later giving a series of interviews.

Gov. Paterson plans to announce his selection on Friday or Saturday. The governor has been coy about his decision, and while he has praised Ms. Kennedy, he has also spoken approvingly of other candidates, including Mr. Cuomo and Representative Kirsten E. Gillibrand, who is beginning her second term as a congresswoman from the Albany area.

Mrs. Clinton resigned from the Senate on Wednesday, immediately after being sworn in as secretary of state.

Some have speculated that with the state facing a $15 billion budget deficit, Mr. Paterson was risking a lot to not select Ms. Kennedy, given her connections to the Obama administration and top Senate Democrats such as Majority Leader Harry Reid. Mr. Paterson appeared to like Ms. Kennedy and saw in her a potential star, but was frustrated and angry by what he saw as efforts by her supporters, especially within Mayor Bloomberg’s administration, to create a sense of inevitability about her candidacy.

”She has pluses and minuses,” Mr. Paterson said in a radio interview earlier this month, adding that Ms. Kennedy ”has a tremendous relationship with the president and that’s certainly a plus. She does not have much legislative experience, which is a minus.”

There have been conflicting signals about whether Mr. Paterson had settled on Ms. Kennedy for the job. Mr. Paterson said earlier this week that he had chosen someone, but some advisers, as recently as Wednesday, remained convinced that he not yet made up his mind on whom to pick.

One close friend of the governor’s said on Wednesday afternoon that “I would be totally shocked” if Mr. Paterson did not pick Ms. Kennedy.

“If he doesn’t go with her, how angry is the Democratic leadership going to be with him?” the friend said.

Ms. Kennedy’s departure instantly resets the political calculus among the remaining contenders, about half a dozen of whom are likely to be serious prospects if Ms. Kennedy is out of the picture. Publicly and privately, Mr. Paterson has talked about the importance of selecting a woman to replace Mrs. Clinton, which could boost such candidates as Ms. Gillibrand, Representative Carolyn B. Maloney, and Randi Weingarten, the president of the United Federation of Teachers.

Mr. Cuomo remains a top contender for the job. He would be unlikely to face a primary challenge in 2010 and has already proved himself as a statewide office holder, and Mr. Paterson is said to be eager to pick someone who will add political and fund-raising heft to the statewide ticket in 2010.

David M. Halbfinger contributed reporting.