BANGKOK, Thailand (AP) _ Customs agents seized 1.4 tons of heroin hidden in bales of rubber sheets bound for New York in what is believed to be the world's largest heroin bust, officials said Saturday.

The 2,822 pounds of No. 4 grade heroin with a street value of $2 billion, were aboard a freighter at Klong Toey Port in Bangkok, said Viroj Laohaphan, director general of the Customs Department. No. 4 grade heroin is 97 percent pure.

Viroj said the drug was discovered when customs officials, acting on a tip, began opening the 665 bales earlier this week. Heroin had been found in 62 bales by the time the search was completed Friday night, he told a news conference.

''The packing in rubber sheets prevented police dogs from smelling the heroin,'' Viroj said. ''If this amount of heroin had left Thailand, it would have killed an uncountable number of people.''

Viroj said Thai authorities have questioned an employee of the shipping company, but made no arrests.

The ship was to leave for New York later this month, authorities said.

Customs officials, who spoke on condition of anonymity, said a rainstorm Thursday aroused suspicions about the cargo. The officials said nothing was found until officials noticed the white powder seeping out of bales wet with rain and noticed that some soaked bales weighed more than others.

Viroj said the previous largest heroin bust was of 2,640 pounds, seized in France in 1972.

Ross Petzing, spokesman for the U.S. Embassy in Bangkok, said the biggest previous heroin seizure that U.S. Drug Enforcement Administration officials in Thailand could recall was a haul also of 2,640 pounds.

That seizure was made several years ago on a shrimp boat off the coast of a Central American country, Petzing said. He said he had no further details of that bust.

Petzing said No. 4 grade heroin, cut, diluted and sold on the streets of New York, would fetch about $1.6 million a kilogram, which equals 2.2 pounds. He quoted DEA agents as saying the drug probably was bought in northern Thailand for about $4,400 a kilo.

The officials said the heroin was believed to have come from Southeast Asia's ''Golden Triangle'' - a major drug producing region where the borders of Thailand, Laos and Burma converge.

He said the rubber bales were to have been exported by the Rubber Plus Import-Export Company to the A.I.C. Company in the New York City borough of Queens. He said U.S. authorities had inspected 300 bales at the company's Elmhurst, Queens, warehouse but did not find incriminating evidence.

Viroj said he received information, however, that a woman representative of A.I.C. was arrested.

News reports said the manager of the Rubber Plus Import-Export had disappeared and police found its offices in Bangkok deserted.

Thai authorities seized 1,496 pounds of heroin aboard a trawler off the southern province of Chumphon last Oct. 20 in what was then described as Thailand's biggest heroin bust.