Mayor Michael Bloomberg is almost certain to face a legal challenge if he tries to alter the city's term-limits law and seek four more years in office.

Several lawyers and government watchdog groups said Wednesday they are mulling legal action to block any changes without the approval of voters, who passed a two-term cap by referendum in 1993.

Bloomberg was expected to announce Thursday that he will ask the City Council to pass a bill giving him and other officeholders the option of running for a third consecutive term.

Even before the specifics of the mayor's plan have been revealed, the idea has already inflamed some critics who are promising a fight.

Public advocacy lawyer Norman Siegel said he has received calls from several people urging him to file a lawsuit, including a political candidate whose campaign plans would be disrupted by a change in term limits.

"The legal question is, can you undo a public referendum by legislative fiat?" Siegel said.

He promised "a hard look" at a legal challenge, a vow repeated Wednesday by other attorneys.

"Lawyers all around the city are going over this with a fine-toothed comb," said Gene Russianoff, a senior attorney for the New York Public Interest Research Group.

Veterans of similar fights, however, say Bloomberg's opponents might not find much solace in the courts.

State and federal judges in New York have a history of rulings that would seem to affirm the City Council's authority to extend or repeal term limits without going back to the voters.

A state appeals court ruled in 1961 that Buffalo's City Council could legally repeal voter-approved term limits without holding a new referendum.

A lower-level appeals court backed New York's City Council when it made minor alterations to the term limits law in 2002 to erase a quirk that would have limited some of its members to no more than six consecutive years in office, rather than eight.