Boxer Mike Tyson is at the center of a new battle between two soft-drink heavyweights, with Coca-Cola claiming that Diet Pepsi's ads featuring the champion are misleading.

Coca-Cola Co. has asked all three television networks to pull a 30-second spot showing Tyson telling reporters there was no question who was going to to win the big fight, meaning Diet Pepsi beating Diet Coke.Pepsi's new advertising campaign built around Tyson says that Diet Pepsi "beat the taste of Diet Coke" in consumer taste tests-- but Coke says indignantly that is not true.

"We couldn't let that claim go unchallenged," Carlton Curtis, vice president of Coke's corporate communications, told Reuters.

At stake is a fast-growing part of the massive $40 billion market for soft drinks. While Pepsi has a long way to go to catch up, statistics show its diet cola is gaining market share.

For the month of April 1988, Diet Coke and caffeine-free Diet Coke had 10.1 percent of the soft-drink market share based on supermarket sales, according to Beverage Digest, while Diet Pepsi trailed with 6.9 percent of the market.

Tyson beat Leon Spinks in a 91-second match on June 27 in Atlantic City, N.J. It was one of the most hyped matches in boxing history with Pepsi as the exclusive sponsor.

All three networks told Reuters the Coke challenge is under review, a process that could take two to four weeks.

"Pepsi still hasn't produced their research. Their claims are unsubstantiated," Curtis said.

However, Stuart Ross, manager of Pepsi-Cola Co.'s public relations in Purchase, N.Y., said "Our research is fundamentally sound. It's based on long understood sampling practices, and we stand behind it."