Discus thrower Ben Plucknett of San Jose, Calif., was stripped of his world record and banned from track and field competition for life yesterday for using illegal steroid drugs before a track meet last winter.

John Holt, general secretary of the International Amateur Athletics Federation in London, announced the ruling, confirming that routine urine tests taken at the beginning of the Pacific Conference Games in Christchurch, New Zealand, in February showed that the 27-year-old American had taken muscle-building steroids. In addition, the IAAF disqualified Gael Mulhall, an Australian woman discuss thrower and shot putter, for life on the same grounds.

A short statement issued by the IAAF said that samples taken from the athletes in Christchurch revealed that "a measure of anabolic steroids were present." The results of the tests were confirmed at a European laboratory in the presence of U.S. and Australian sports officials, the IAAF said.

The IAAF also invalidated all competitions in which Plucknett has participated since the New Zealand meet, forcing him to forfeit his world record. He first set the mark of 233 feet 7 inches at Modesto, Calif., on May 16, then raised it to 237-4 at Stockholm last week.

According to IAAF rules, Plucknett and Mulhall may appeal the lifelong ban from track and field competition in 18 months. In previous cases, the IAAF has restored eligibility, even though it was critizized for doing so in 1980, when several banned Iron Curtain athletes were allowed to compete in the Olympic Games.

Plucknett, ranked sixth in the world last year, added 10 feet to his all-time best with his world-record performance at Modesto. Even he expressed surprise at the time, saying, "It shouldn't really have happened. You just don't jump 10 feet over your previous best. You don't jump from 11th all-time to No. 1."

The world record now reverts to Wolfgang Schmidt of East Germany, who threw the discuss 233-5 in 1978.

Plucknett, who was in Dublin yesterday for an international track meet, could not be reached for comment but was reportedly conferring with his U.S.-based lawyers via telephone.

Plucknett's coach, Pete Petersons of Santa Barbara, Calif., was with the athlete when Dublin track officials told him he was not eligible to compete in the Irish meet, which begins today.

"His reaction was one of disbleif," said Petersons, who leveled strong criticism at the IAAF and The Athletics Congress for not notifying Plucknett of the ruling privately. "How can the IAAF make such an arbitrary ruling without notifying him in person . . .?"

Petersons said that although it was too early to know what Plucknett would do, he speculated that the IAAF ruling left "a lot of grounds for appeals and lawsuits."

Officials at the Indianapolis-based The Athletics Congress, the federation that oversees U.S. track and field, said they were distressed but not surprised by the IAAF ruling.

"The fact is we'd been notified sometime ago that initial tests were positive and that more tests would be done," said spokesman Pete Cava. "You hate to see this happen . . . especially when Plucknett was having such a great season."

Anabolic steroids, the type reportedly found in Plucknett's urine, are believed to build muscle mass.

The 1981 Amateur Athletics Annual lists Plucknett as standing 6-foot-7 1/4 and 273 pounds. However, Track and Field News reported that Plucknett weighed 309 pounds the morning he set the record in Modesto.