Mayor Bloomberg, who won a reputation as an anti-tobacco activist in his first term by raising cigarette taxes and banning smoking in restaurants and bars, is now vowing to veto two anti-teen-smoking proposals that are before the City Council.

City Council members yesterday expressed surprise at the mayor’s position. “I’m speechless. I am speechless,” said Council Member James Gennaro, a Queens Democrat who is the sponsor of a bill to increase the minimum legal age to buy cigarettes in the city to 19 from 18. Mr. Gennaro pronounced himself “flabbergasted.”

A second proposal, sponsored by the Democratic leader, Joel Rivera, would go further, banning the sale of cigarettes to anyone younger than 21.

A third proposal, which the mayor did not threaten to veto but which his administration criticized, would ban the sale of flavored tobacco products, such as Kool’s “Mintrigue” and hazelnut-flavored pipe tobacco.

Mr. Bloomberg and his health advisers say similar restrictions on cigarette sales to teenagers in places like Nassau and Suffolk counties haven’t proved effective. And the advisers expressed concern about the effects of the measures on 18-, 19-, and 20-year-olds who are already addicted to tobacco and now would not be able to get their nicotine fix legally.

An assistant health commissioner for tobacco control, Sarah Perl, said increasing the minimum age would increase cigarettes’ “forbidden fruit” appeal, which she said tobacco-industry documents suggest could make smoking more alluring to young people.

“Sending the message that ‘smoking is an adult choice’ may paradoxically play into the hands of the tobacco industry and attract more children to tobacco,” Ms. Perl said.

Ms. Perl said 11% of city public high school students smoke cigarettes, less than half the national average of 23%. She said the most effective way to combat smoking is by changing the perception that smoking is socially acceptable.

Mayor Bloomberg, who said at an afternoon news conference that he would veto the two age bills, said he and the city health commissioner aren’t convinced that the council’s latest antismoking initiatives would work.

“The best ways to reduce smoking among young people is to raise cigarette taxes because cigarette taxes have been shown to directly impact the younger people’s ability to buy cigarettes,” the mayor said.

The city forecasts that tobacco tax revenue for the fiscal year 2006 will be $121 million, a spokesman for the mayor, Matthew Kelly, said last night. State and local excise taxes are $3 a pack, and the mayor has proposed an additional 50 cent a pack increase. In addition, cigarettes are subject to the sales tax.

The opposition of the mayor — who has donated at least $125 million of his personal fortune to global anti-smoking causes — surprised many on the panel yesterday, who questioned whether other factions in the administration were behind the opposition to the bills.

“I find it hard to believe that this testimony actually comes from you because I know what an ardent advocate and fighter you are against the use of tobacco, particularly among our young kids,” Council Member John Liu of Queens said to Ms. Perl.

Mr. Gennaro suggested that the Finance Department might have influenced the decision.

Physicians from tobacco-control groups, including the American Cancer Society of New York and New Jersey and the American Lung Association of the City of New York, testified in favor of the bans.

The only council member during the bulk of the hearing who questioned the wisdom of the bans was Simcha Felder of Brooklyn.

“It’s not popular to say anything against a bill that would be prohibiting smoking,” Mr. Felder said, adding, “Take a census, certainly from the 18-, 19-, 20-, 21-year olds, and they’ll say to you, ‘if we can go out to war and potentially be killed we should have the right to buy a pack of cigarettes.'”

Max Neuman, a teenage smoker waiting for friends on Chambers Street where he attends Stuyvesant High School, said that he and his classmates who smoke would always find a way to get cigarettes no matter what officials try.

“People are going to smoke. People have smoked,” Mr. Neuman said as a lighter dangled from his belt loop. “There’s no law they’re going to make that’s going to stop them.”

But the City Council members who support the bills argued that with more than 400,000 Americans dying of tobacco-related disease every year, the need to curb underage use is so great that it’s worth trying anything that could make a difference.

“Most things are done by theorizing, not done with hard core facts,” Council Member Rivera said yesterday, “because if we wait for facts for everything we will never have a law enacted in the City of New York ever.”