NEW YORK (AP) _ For the agency Backer Spielvogel Bates, the loss of the $100 million Miller Lite advertising account tasted anything but great.

Miller Brewing Co. announced Wednesday that it was reassigning the Lite beer account to the agency Leo Burnett in Chicago. The account, worth an estimated $100 million in ad billings, had been held by Backer and its predecessor agencies ever since the brand was launched in 1973.

Key executives at Backer helped ignite the ''Tastes Great, Less Filling'' ad debate that helped make the brand the nation's best selling light beer.

''Obviously we are very disappointed,'' said Dean Scaros, president and chief operating officer for Backer Spielvogel Bates, New York. ''You develop an emotional attachment to the business and the brand.''

Burnett and Backer have each handled numerous assignments for Miller brands over the years. Backer will continue to create advertising for the Miller Genuine Draft and Genuine Draft Light brands. Burnett continues to handle Miller High Life advertising and smaller brands.

''This move allows Miller to take full advantage of the resources and creative talents available at two of the world's premier advertising agencies,'' the Milwaukee-based brewer said in announcing the changes.

It said the switch will allow Backer to focus full attention on the Genuine Draft accounts, including the national rollout of Genuine Draft Light in April.

Industry sources say Miller may spend close to $100 million advertising those brands this year.

But some beer industry analysts said Miller Lite, which trails only Budweiser among the nation's best-selling beers overall, has lost market share in the light beer category to the fast-growing Bud Light and Coors Light.

''They may be looking for new blood to make the brand go,'' said Jerry Steinman, who heads Beer Marketer's Insights in West Nyack, N.Y.

The loss of the Miller Lite account closes one of the longest-running relationships between a brand and an ad agency in the beer business.

Backer Spielvogel Bates' Bill Backer and Carl Spielvogel were at the ad agency McCann Erickson Worldwide that created ads for the national launch of Miller Lite beer in 1973.

At the time, light beers were seen as diet drinks that appealed mainly to women. But the agency came up with an approach that featured retired athletes and other celebrities argue over whether they drink Lite beer principally because of the taste or because it didn't leave them feeling bloated.

''That was a way of communicating that Lite beer had low calories but it was also a beer for real beer drinkers,'' said Scaros.

Backer and Spielvogel along with Bob Lenz, Steve Leff and others created the ad agency Backer & Spielvogel in 1979 with Miller as their first client.

The agency has held onto the account even after it was acquired in 1986 by the British holding company Saatchi & Saatchi Co. and its merger a year later with another Saatchi acquisition, Ted Bates Worldwide.

Scaros declined to speculate about why his agency lost the Lite account. ''They made a business decision,'' he said.

But he said the agency has ''enormous continuing responsibilties with Miller.'' Neither Miller nor Scaros would comment on Miller's spending.

He said Miller remains the agency's biggest account in the United States, where it handles more than $1 billion in ad billings a year for clients such as Mars candy, Hyundai automobiles, Campbell soups and Wendy's hamburgers.

Genuine Draft was the ninth best-selling beer last year at an estimated 5.8 million barrels, according to the trade publication Impact.

The best seller was Budweiser at about 48.5 million barrels, followed by Miller Lite at about 20.2 million barrels, Impact said. It said the third best-seller was Coors Light at about 12 million barrels and Bud Light was next at about 11.8 million.

Steinman said light beer shipments rose about 6.4 percent last year compared with less than 3 percent growth for the overall beer market.

But he said Lite's share of the light beer segment slipped to 35.2 percent from 37.1 percent in 1989, while both Bud Light and Coors Light shares rose.

Lite beer has been relying less frequently on retired athletes and the never-ending argument over why to drink Lite in its ads. The brand ran a celebrity-less ad campaign leading up this year's Super Bowl, for instance, tied to a contest for free tickets to the game.

Susan Henderson, a spokeswoman for Miller, said she could not discuss future advertising plans for the brand although she did say a new wave of ads with ex-baseball player Bob Uecker and others would soon air.

When asked whether the celebrity approach and the debate would be dropped with the new agency, she said, ''We are going to build on the heritage and equity of the Lite brand ... (and) use those elements as appropriate.''