The city comptroller, William Thompson Jr., began contacting supporters yesterday to tell them he has decided not to run for mayor next year. News of Mr. Thompson’s decision was first reported in The New York Sun on Tuesday. He will make a formal announcement at an event Monday, aides said.

Aides began calling reporters yesterday to tell them of Mr. Thompson’s decision to remain at his current post. “This is a job that Bill Thompson enjoys very much,” an aide said yesterday in a telephone interview.” This decision has been a very personal one for him. He is very proud of his successes over the last three years and he wants to continue them.”

The comptroller laid out parameters that he had said earlier in the year would be paramount in making a final decision. He said that if Mr. Bloomberg’s poll ratings were low and if he disagreed with the direction the city was headed, he would throw his hat into the ring.

“With Billy Thompson out of the race you have a bunch of candidates breathing a sigh of relief,” a Baruch College professor of politics, Doug Muzzio, said.

With Mr. Thompson out, political analysts are assessing the implications for two of the city’s best-known female politicians: the Manhattan borough president, C. Virginia Fields, and the chairwoman of the council’s education committee, Eva Moskowitz.

Ms. Fields has said she may run for mayor, and Mr. Thompson’s decision means she will have less competition for support among their fellow black voters.

Ms. Moskowitz has said she’ll run for either comptroller or Manhattan borough president, a job Ms. Fields must leave because of term limits. “I can’t see Eva taking on Thompson, so his announcement will close a door for her,” Mr. Muzzio said. “The one this really helps is Virginia Fields, because she won’t have the same competition for the black vote. If Eva decides she is leaving the council, then the next logical move is to run for borough president. She has a shot to win that.”